The original intended audience for Gundam was supposed to be college students if I remember correctly and not highschoolers. 1976 was the real start of when you had this massive wave of engineering finesse in Japan that overtook everything else in the world. It was the time when Japan was forming an obsession with mechanics, with model kits of everything from fully articulated 1:36 scale 50cc scooters to giant 1:20 scale warships that would take up an entire table. Kids couldn't afford these models as they were priced strictly for adults.

Gundam definitely fit into that "engineering fantasies for young professionals" niche, at least until ZZ came around in 1985. Gundam has the root word of "gun" because they were originally these more grounded fantasy weapons instead of man made demi-gods that appeared in shows like UFO Robot Grendizer. They weren't supposed to be superheroes, they were what engineering minded young men thought would be cool to have if they were given an unlimited budget to create bipedal tanks that could do the job of bomber aircraft, navy destroyers, and orbital bombardment satellites all in one. That's why Gundams, especially Zaku units, move slowly, pivot in unnatural ways, and use jets and wheels for locomotion, because they're giant tanks with manipulators that hold guns and not suits of armour. BattleTech also comes from that same origin, although it and Mechwarrior's development went all in on the "tank but with legs" idea instead of slowly losing their identity to the super robot genre.

The melodrama they mixed in as framing to discuss Japan's post-war military pacifism was incidental to creating and populating the backstory for an engineer's dream unlimited budget mobile weapons platform. So they weren't the Marvel equivalent back in 1979, they were more like Japan's answer to some of Robert Heinlein's militaristic concepts in Starship Troopers and The Moon Is A Harsh Mistress, where the concept came first and the story was just an excuse to see that concept in action.

G Gundam and SD Gundam are more like the Marvel movies, in that they strip away most of the issues being discussed and coast on the aesthetic similarities and caricaturized versions of themes from the source material.

This is a good rundown of (the history of) the appeal, particularly to male viewers. I hesitate to call the melodrama "incidental", though, as the female viewers it drew in were the ones who saved the franchise (per Tomino) when it initially failed to take off. The creators recognized where their bread was being buttered, which is why so many series in the franchise (including the ones most grounded in some semblance of mechanical and military knowledge) end up centering around either love stories or a troupe of unusually handsome young men.

That was half the equation; the other half being the transition from toy-based to model-based merchandising, as you said, which drew back in the male fans.