Gravity is directly related to Mass, so I am not sure I understand how you are separating the two. Or I have been out of college for too long and don't know what I am talking about...lol...
Yes but what we are talking about is the propulsion method and where the momentum comes from. That's likely Newtonian - even with a "reactionless" drive, there may well be a Newtonian reaction mass: but it not carried onboard the vessel like a torch or rocket ship, rather it is being exerted on another nearby mass through some coupling method - like a bicycle or road vehicle in physical contact with the ground. At least with the right ballast and tread a road vehicle would generate the same
or very similar motive thrust on any size planet regardless of planetary mass/gravity. The reaction momentum experienced by the coupled body is the same in absolute terms and while almost undetectable for planetary sized body if the coupled reaction mass was something small like an asteroid then it would show a detectable acceleration in the opposite direction of the vehicle movement. But the acceleration of the vehicle is the same for the energy it expends regardless of whether it is pushing against an apparently motionless planetary body with detectable gravity or a rapidly receding rock with no detectable gravity. What matters is that the vehicle and the mass it leverages are coupled together so that they can exchange force.
But there is no reason that the (fictional) coupling mechanism is through a gravity field, that notion seems to stem from people not distinguishing between gravity, contra-gravity, artificial gravity and anti-gravity and because it maybe seems obvious because presence of mass is associated with the presence of a gravity field (which notionally transfers force between masses). But in fact general relativity gravity does not really work like a normal force field so we actually have to invent something fictional for the job instead. Other fields such as electromagnetic and nuclear forces exist and have interesting properties to borrow for our coupling force but all you basically need is something magical that transfers momentum between two spacially separated masses.
In other words Traveller is basically assuming the existence of some currently unknown force (or several) that can be artificially generated and couples with mass (like gravity) but can also be used as an attractive (artificial gravity), repulsive (lifter plates) or orthhogonal (thrust plates) force, or all three simultaneously (like electromagnetic fields on steroids) and may, in some circumstances, have an exponential coupling distance cut off (a bit like like nuclear forces).