Quantify what EP a .5G/1G/2G field delivers to the BG per turn...
Presumably one can quantify based on the kinetic energy that would be imparted had the globe not been there.
...Globes are invisible? ...
The original rules as written made them render the ship invisible when on full. Then an errata made them render the ship invisible when set to a flicker rate at which the ship's own radiations matched the background - but that really wasn't workable either because they'd be flickering neutrinos that would give the power plant away and, even excluding that, the ship itself occludes the star field. You'd end up seeing a little piece of the star field that was dimmed due to the black globe flicker effect, at the center of which was a flickering neutrino source with a black ship-shaped shadow that occluded the star field. So, no, not really invisible when you thought about it, but that was what the game declared.
I'm currently running with invisible to ships because ships are not able to examine the entire 360 sphere with the focus needed to pick up an occlusion measured in milli-arcseconds, but visible to starports. Which is probably a stretch too, but the invisible thing does produce some interesting encounters, so I'm not certain I'm ready to let go of it.
Neutrinos? So your own power plant can overload your own field?...
Well, yes, but the neutrinos are the least of your worries. Your own heat output will get you long before the neutrinos get to be a problem.
...Really? Is that in T5 or something?...
That's my inference from the ship not being detectable on mass detectors, which is a sensor tech that shows up in MegaTraveller. Otherwise you don't have an invisible ship - you have a black body mass that's giving off no EM and that doesn't reflect your own active sensor bleeps, and the sensor tech screams, "BLACK GLOBE!!!"
Gravity isn't a force in the same way electromagnetism, strong and weak are.(In point of fact according to general relativity it isn't a force at all).
A ship with a black globe up is still following its geodesic through spacetime and so will be affected by what we perceive as gravity.
And would therefore be detected on mass sensors, and is not invisible.
Gravity is a fundamental interaction. The four fundamental interactions are quite different from each other, but three of them are carried by a particle - photons for EM, W and Z bosons for the weak, gluons for the strong. Science has been expecting gravity to be mediated by some particle as well, but we haven't found it yet and alternate theories suggest there isn't one and gravity works like you're describing. However, the game subscribes to the gravity particle concept, as near as I can tell.
The field clearly stops photons, it clearly stops hadrons, and it clearly stops mesons, so it seems to be stopping everything quark-based. Not completely sure, we don't have anything in the game like a boson or gluon emitter - unless that's how the nuclear damper works - but it seems to be a reasonable assumption. Gravitons are thought to be bosons - if they exist - so would likewise be affected.