Damage types.
Silly, pedantic, element-based damage types.
One thing about role-players is that they fetishize damage types far more than they should. Science fiction games often fall into this trap, but you see this in 5E with all the damage types there, with 13 types being in the game.
Starfinder had eight types, but weapons have levels, and every three or four levels, every weapon type was repeated, with most of the damage types represented. So there would be eight level 1-3 pistols, eight level 4-6 pistols, and so on. Sometimes you needed a cold pistol, other times you needed that electricity pistol. Starfinder's weapon lists were massive, easily filling hundreds of entries, and adventure paths would add dozens more weapons for no good reason.
The game also had two armor class values, energy and kinetic.
The Esper Genesis game falls back on the 5E tropes, but unless a monster has a resistance or weakness to that type of damage, the damage is just damage, and who cares? You have one AC value. Of all the science fiction games, this one does things the most "5E way," and you are not worrying about damage types for most attacks.
And we get to Star Frontiers. You can combine a suit and a screen to layer defenses against different types of damage. Of suits, you can do with either laser or ballistic/melee protection. With screens, you get protection against laser, ballistic/melee, electrical, and sonic attacks. Typically, all our characters wore suits that provided ballistic and melee protection, as well as a screen for lasers. This way, you could rely on the suit for protection in more social situations, such as a fight in a space bar, and then, when the lasers started flying, the screen was turned on. Electrical and sonic attacks were rarer, so those tended to be the ones that got through your defenses more often.
Star Frontiers combat meant "turning on your shields" in personal combat, and making sure you had the correct resistances to damage types in your group. GURPS really isn't about that, and it is a different style of combat in the system that is more armor-focused. We have force screens in Ultra Tech that can function similarly, with a DR 60 (for personal conforming, TL 12^, super science), and you can specify an energy type for a 50% cost reduction. Star Frontiers is not a TL12 setting, though. For a TL 11 field, I would halve the DR to 30.
The conversion is at a point where, if I try to convert mechanics, I will be here doing these little conversions forever, trying to tweak it and make it work, but inevitably breaking something else. One of the things is that GURPS has solved many science fiction gaming problems with the rules the game already contains. GURPS is a well-tested and well-designed system that works, taking into account numerous science fiction tropes and pieces of gear and technology.
I am not giving up on this, but if I want an exact simulation of Star Frontiers, that would require a significant amount of design work. For most people, just use GURPS Space and flavor the setting with the Star Frontiers races and setting, while using GURPS to replace the gear and tech lists entirely.
For the average game, tossing out the Star Frontiers gear, which TSR designed for their "attack versus defense" game, is the way to go. GURPS is far better designed, written, and put together for science fiction gaming the way it is, and more people should just use the races and universe.