I've found this a fascinating discussion.
Let me say that I have no direct military experience - but have studied the military history (and unit organisations and performance) of the 17th, 19th and 20th centuries, and reckon that I am probably as well placed as most to try to predict what future military history might look like.
It seems to me that the important thing is to appreciate that there are, and always will be, two essentially different kinds of offensive military operation which, for convenience, I shall term the RAID and the INVASION.
In a raid, the object is to insert your troops, carry out some specific mission, and extract your troops again at the end of it. In an invasion, your objective is to take and hold your enemy's real estate and divert its resources to your benefit rather than those of your enemy; and to achieve this you will need to eject, destroy, or disarm the military forces which are available to your enemy to defend it (or, most likely, some combination of all three) - following which you look to establish an army of occupation sufficient to prevent renewed resistance and keep the civilian population subdued to your will; and establish a civilian administration sufficient to harvest the resources that the territory has to offer and turn them to your advantage.
I imagine that in interplanetary and interstellar warfare, raids will always be the preserve of the Marines. They require close co-operation with the Navy (whose job it is to inserts the troops, and subsequently to retrieve them) and this is what the Navy does. Hoever, every raid is unique in terms of objectives, size of force required, and skills and equipment called for: and I therefore envisage raiding forces being ad-hoc organisations, put together with whatever resources are available at the time. In practice, these will frequently be the organic marines units carried aboard the navy's fighting squadrons; and IMTU it goes without saying that these units train not only in offensive and defensive boarding actions, but also in working with the marines contingents from other ships of their squadrons in raiding operations.
Turning, then to invasions - it seems to me that the role of the marines is to be the spearhead units, establishing the initial planetary bridgehead. I envisage this being a role which combines elements of both the USMC at Guadalcanal, and the German fallschermjaeger at Crete or the British and American paratroops in Operation Market Garden. They need to get down fast, they need to sieze and hold a piece of ground sufficient to bring in the subsequent surge of Army personnel required for a breakout; and the Army then take over for the breakout and subsequent fighting operations.
IMTU the principal "mud marines" unit is the Assault Division. This consists of a Light Brigade and two Regular Brigades. The Light Brigade is the most interesting, as it is the unit which is "on point" on any planetary assault. Its objective is to get down and establish a secure perimeter into which the two Regular Brigades of the division can deploy. It is not expected to do too much fighting because the idea is to establish the perimeter AWAY FROM any opposing troop concentrations. Its job is to overwhelm the defences of the chosen perimeter / deployment area, and hold until relieved.
The Light Brigade consists of three battalions, which are all released from orbit and descend in their own particular manner using gravitic devices. They do not use armoured gravitic vehicles - these come in with the Regular Brigades. The Light Brigade is unarmoured, and is protected in its descent by the Marines Fighter Squadrons carried by the assault ships, and by the element of surprise / recoil from initial operations aimed at suppressing defensive aerial capability. And of course, until they descend and establish their initial perimeter, the defenders don't know WHERE on the planet they are aiming to deploy, and so cannot really defend effectively against the initial landing because "to defend everywhere is to defend nowhere".
The three battalions of the Light Brigade are the Jump Troop Battalion (equipped with vacc suits and grav belts, which descend as a swarm, much as paratroops do); the Speeder Battalion (which is the Assault Division equivalent of the German motorcycle battalions of WWII - fast but very lightly armoured and equipped - using their vehicles for mobility but fighting on foot) and the Air/Raft Battalion - which is much the same in concept as the Speeder Battallion but with a different balance of mobility to manpower, and which has a number of heavy support air/rafts with VRF gauss guns on pintel mounts to lay down supporting fire as necessary.
Even with the availability of gravitics, I do not see the elimination of ground combat between infantry and their supporting units. It is ground that you are trying to take or defend; and ultimately it is troops on the ground who occupy and take ground. You can move those troops around in wheeled vehicles, tracked vehicles, or flying vehicles (whether fixed wing, rotary wing or gravitic) but at some point they will always need to get their feet back on the ground and that is how they will do most of their fighting. Advances in the manner of transportation, and the armouring and arming of the transport, will not affect this universal truism.
I do not think that the availability of gravitics will take the battlefield off the surface and up into the sky. There is nowhere to hide up in the sky. Nowhere to protect yourself and give you an advantage over your opponent by getting "hull down". And flying craft will always be vulnerable to ground fire. The technological balance may shift this way and that - but high in the sky you can be detected. And if you can be detected, you can be shot at.
There will still be aerial combat, of course, because control of the skies will be still be essential if you are to give your ground troops air support and deny it to the ground troops. But the simple fact is, and always will remain, that it is troops on the ground alone that can control territory and divert its resources to the use of their side not the enemy. And the aerial battle will always be - as it has always been - an adjunct to the ground battle not a replacement for it. Moreover, never forget the lesson of Dien Bien Phu - air mobility can be a very double-edged sword, and can be turned against the side that uses it if they do not also maintain a sufficient presence on the ground. The VK learned this lesson and took it to heart. Did the US?
I have also given some thought to how gravitics are used defensively. Gravitics can produce positive gravity fields as well as negative ones - and it will always be possible to pack more gravitics into a stationary piece of real estate than it will in a vehicle flying over them. So I see defensive "gravity fields" being the high-tech development of the minefield.
Think about it.
You have a fortified position, which the enemy need to take out. You surround it with a ring of gravitics creating a localised POSITIVE gravity field of 3G or thereabouts. Anything trying to overfly that - whether conventional or gravitic - is going to come to earth with a bump. The only way to avoid this - assuming you know it is there - is to "go high". But at high altitude you are detectable and targetable. In this way, gravitics can protect against low-level stealth attacks.
The gravity field will also be a formidable obstacle for the attaking ground troops. It will ideally be in a cleared area with a good field of fire - a killing zone. Because when those ground troops hit it (and it may not be switched on until they are mid-way across it) they are going to wallow. It'll be like advancing through treacle. And taking them out ... well, it'll be like taking candy from a baby!
Alternatively, a NEGATIVE gravity field can play merry hell with an attacker, too. Flying low - hugging the ground to avoid detection by radar or whatever other detection devices are in use at those tech levels - you overfly a repulsive gravity field and suddenly you've popped up to an altitude of a couple of hundred metres where you can be detected and targeted. Not nice!
And as for the anti-gravity anti-personnel mine ... you tread on it, activate the repulsive gravity field, and you're thrown 50 metres into the air. You're unlikely to survive the descent from that height ... and even if you do, you're unlikely to be fit for further action. And, unlike 20th and 21st century explosive mines, these are not a single-use weapons. They can throw as many people as tread on them into the air. So combat infantry, I imagine, would need to advance behind combat engineers with gravity detectors.
Then there's the oscillating gravity field, the purpose of which is to neutralise slug-throwers and other non-energy weapons. Again, these are set in advance of your positions - and the further in advance the better so long as they are in a position where incoming bullets and other projectiles have to overfly them. If you do not know the gravity over which your projectile must fly, then you cannot aim it effectively. If the gravity oscillates, then there is no way you can know it and your bullets are going to fly wild.
Yep - the need to occupy and hold real estate would be constant; but the appearance of warfare would be significantly different.
