So we know Gorn capture other species, pit them against the Gorn and each other ala the strongest M&M copypasta, and then send the strongest M&M back to M&M Mars space. La’an was the strongest M&M, for example.

I tried to touch on this in my OP, but the problem with this is that unless Gorn generations are very spread out (unlikely, given the rapid gestation period and rather ad-hoc method of implanting eggs), the odds of a "superior" elder beating out all of it's slightly younger competition remains quite slim. This is a brawl, not a neatly organized bracket, and random chance will invariably play a big role in who wins and who loses. The top Gorn from generation A suddenly finds itself as merely a slightly advantaged individual in a whole new field of competitors the overwhelming likelihood is that one of them will prove the ultimate winner, and then suffer the same fate. If the "strongest M&M" were thrown into a fresh bracket instead of being mailed to the parent company, it's almost certain to be toppled. You wind up with a species whose "true" lifespan and adult form is irrelevant, because every individual dies long before they come anywhere near adulthood.

More importantly from an evolutionary perspective, though, the success or lack thereof of an individual Gorn has almost no effect on their ability to reproduce. All the Gorn need, apparently, is to survive long enough to spit on a viable host. Anything that happens after they do that is irrelevant, and thus won't be selected for evolutionarily. And it strikes me as highly improbable that growing to much greater size and having enormous strength (never mind developing sapience) are unlikely to emerge by pure chance without evolutionary pressures making those traits more likely to be passed on.

The "Strongest M&M" problem is probably mitigated by lower density of baby Gorn in the wild than what we've seen on screen so far. If a brood typically manages to winnow itself down to a single individual before any of them can spit on a host, and the hist typically has enough time to travel somewhere else before the next generation hatches, then you have a situation where the strongest of the babies will generally reproduce and then generally have the chance to continue growing into "true" adulthood. Unfortunately that still doesn't answer my second question of how that adult form evolved at all when it's very existence has no clear benefit to the animal's ability to reproduce.