By the time I started playing in the early 80s Metamorphosis Alpha had been and gone. Gamma World and and Boot Hill I initially knew about because of mentioned in the 1st edition DMG. From what I remember GW had races of a sort - Pure Strain Humans, Mutants and something in between, but it didn't have classes, instead you gained powers from random mutant abilities. Boot Hill didn't have hp, so was highly lethal. Can't remember if it had anything resembling classes.
Call of Cthulhu came in quite early, along with Mutants and Masterminds and Champions (both superheroes). On the trail of those Games Workshop released Golden Heroes, which was kind of a "5e" version of Champions - much simpler and more playable. Oh, Star Frontiers (space opera, lighter than Traveller) was out around then too.