The handwavium for how gravitics works IMTU.
Some kind of power source that creates a field which reacts with gravity. The polarity of the field can be adjusted such that the gravitational force is applied differently.
Earliest/cheapest grav tech can simply reverse the field so gravity provides lift.
First gravs were: large external power source, sea level, external power applied to field generator attached to a chassis, result the chassis rising above the ground and hovering – hover sled.
Over time improved: power source small enough to be carried on the chassis, solid hover ability, separate source of thrust – hover tank.
Over time improved: field generates multiple of G force so can lift rapidly also field can be angled to give thrust and lift – flying tank.
Over time improved: full air/raft functionality.
Re-conditioned grav components from scrapyards are jury-rigged into black box components on systems like Glisten for sale on backwater planets. The advanced controllers are generally removed so components only provide lift but they can be bolted onto platforms and switched on and off which is the minimum needed. Grav parts like this are used on hundreds of lotech worlds with the thrust supplied in hundreds of different ways including animals, steam engines, wind etc.
Basic reconditioned grav parts and spare parts for other tech is a very common Free Trader cargo.
A lot of backwater systems have a hybrid kind of gravpunk technology where the cheapest kind of grav tech is combined with local tech capability so John Carter on Mars type planets can exist in the remote parts of the 3rd Imperium where the dreadnought fleets never go.