In this instance, it's useful to examine the specific strategic picture.
The Imperium's depots - the best place for a military gate - are very well defended and set back a ways from the usual fronts. Presence of a gate would make them impossibly well-defended: opposing fleet jump in, and the entire strength of your empire pops in a few turns later while the local defenders struggle to hold the attackers at bay. Add to that the fact that the opposing fleet has a number of star systems to get through before it gets to Depot, and it becomes pretty much impossible for the opposing force to defeat a Depot without doing something beforehand about the gate or the fleet that's going to be coming through it; there's no hope for surprise.
I'd need to know some more details before accepting (or refusing) your picture:
Woould those gates allow for contiuous transit (as a bridge would) or would they have to be activated for every ship (as a ferry would)?
In the former case, how many ships would be able to cross it (to keep with the bridge analogy, what's the weight allowance of the bridge)?
In the latter case (the ferry analogy), how long between activations (to allow for the gates to cool off, the space to reset after the stress, the power to build up, whatever it be)?
And see that even if news are immediately sent by the gates (Smart money is that courriers are ready at teach gate, but, again, how long to send them to each receiveing gate?), unless the fletes are already assembled in the depots (some of them would be on patrol, training maneuvers, repairing/reffiting/maintenance, etc...), how long to reach the gate and jump to cross it to the battle?
What effect has the gate on the ship, and how long to recover from it (screens must be down, maneuver probably limited, etc...)?
So, I guess a surprise attack on a gate will not see ships arriving to reinforce in a few turns (at least a few days/weeks), probably giving time for the gate to be damaged/destroyed if the incoming fleet is larger (or at least more powerful) than the deffending one.
But, as those gates would not be in front lines, I don't envision a surprise attack on them to be a possibility, so, in the purely strategic POV, your prediction might be valid.
Offensively, it is simply a question of weight and the ability to apply it where needed, when needed. The Imperium is the most powerful of the empires in that immediate region. Its greatest weakness is its inability to concentrate power, owing to the great distances and long travel times involved. This system would allow it to concentrate power, bringing overwhelming force to one small region. As the depots asset back a ways from the front, the effect would not be immediate and surprise could not be obtained. However, if the Imperium is the sole empire with the technology - and if it's TL15 technology, it will have the lead on everyone except those Fungi-from-Yuggoth folk, if I remember correctly - then the lack of surprise will be irrelevant since the Imperium can bring overwhelming force against a single sector while the opponent, even with forewarning, is struggling to bring his forces up to respond.
Even in this case, how long before the enemies at the opposite side of the Imperium realice their frontiers have been weakended and (knowing they might be the next ones) take advantage of it (while the massed fleet is dispersed in offensive opperations along the active front)?
And see that relying on your predictions, it's quite posible that fleet is kept at smaller numbers, as the concentration capability gives a perception that a smaller fleet sufices...
