The model in MT seems to be reactionless thrusters. I guess the 'G' rating reflects how strong of a reactionless push you can get.
You would think mass would be involved, but we never really figure out mass for spaceships.

Even a reactionless push should be quantified in force supplied (the F in F=m a). As was pointed out here, Traveller operates on The Rule of Cool, and physics, math, and reality are not cool. Just handwave in a 0.4g drive and call it good.