The Gundam gets progressive minor upgrades in Jaburo and after Solomon I think. It gradually evolves from Okawara's plain version to something similar to Ver.Ka/post 1995 MG version with added thrusters and such.
There is a Core Booster/G-Fighter equivalent that has a rear platform large enough to have it function as an SFS. There is also a smaller fighter oriented Core Booster IIRC.
The GM doesn't look as dopey and actually scores kills. It looks more like Okawara's RGM-79[G] with GM Command shoulders. Amuro actually gets to pilot one and confirms that its response time is inferior to that of the Gundam. So we can finally confirm that Amuro in a GM is not the same as him piloting the Gundam, in spite of the similar catalog specs.
There are some GM cannon and missile equipped variants based upon the above standard model, but there is also one that seems to be an overall upgrade similar to the GM Kai and designed for ace pilot use, specialised in close combat.
Gouf looks fatter while Ramba Ral is actually slimmer. The Dom has broader shoulders, Z'Gok looks like Z'Gok E, the Braw Bro looks more monstrous with its tentacles, and I think the Gyan had almost no change.
The Gelgoog is a merger of the MG Ver 2.0 with the beam machine gun and added thrusters from the MG MS-14A Ver 1.0. Afterall, the Ver.2.0 was said to be based upon Yasuhiko's illustration.