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.

Maybe, originally on the Gorn home planet, the strongest just keep growing to fend off an ever increasing wave of hostile baby Gorn, becoming more cunning and intelligent on each subsequent generation. Maybe the spacefaring stuff was originally some Gorn thinking "wow this planet sucks, I need to get away from it so I'm not eaten by my own young."

And now, perhaps the Gorn do the same for their terrible cannibal babies - they get pitted against each other in the Baby Gorn Fighting Arena, Sponsored by M&M Mars, and the strongest are kept to become adult Gorn and to be used for breeding purposes and also for spacefaring stuff.

Maybe the Gorn are just trying to help other species become stronger. They are helping to select for the strongest M&M to send back to the factory for breeding purposes, and we just don't understand - cultural differences and all that.

Well you're reasoning based on ancient Darwinian doctrine from the 21st century... That's completely surpassed at the time of SNW!

I mean, not much time later the crew of another enterprise discovers that it's somehow possible to seed the galaxy with... Something that directs evolution towards humanoid races, so....

They need someone to put the babies where they need to be at the very least

Let's imagine the baby Gorn start off the way you describe, a perpetual small hunter that also produces more offspring. As they age they get bigger and stronger before they finally die.

The adults who take care of their offspring have an advantage over adults who do not care for their offspring, and possibly even more over the babies who never become adults.

There is another selection barrier as well. If all you have are baby gorn, what happens when you run out of hosts? This can easily happen if the hosts are over-hunted. If baby gorn pop out and there are no hosts, and they die out in a few years or even months, that's an evolutionary dead end. The ones which can last a long time until new hosts are available will eventually be selected for.

It’s also a lifestyle that selects for intelligence (small hunters tend to be pretty smart)

Humans are already on the large side for mammals and are pretty intelligent (or let’s rather say successful)

Why would there be a species where the adults are intelligent and social enough to be a spacefaring power, and yet apparently nothing they learn as an adult is needed for an individual to pass on it’s genes?

You already do know another species who does that, and you call those useless adults Nana and Gramps.

There’s more to a society (and to evolution) than just surviving and procreating, you need knowledge and history. This you can only build when you’re not constantly fighting for the very survival, so having people around who aren’t busy with procreating all the time is actually the most likely route to developing genuine sapience.

I'm assuming you want a diagetic explanation, not "bad writing"? Because we already know which one it is.

Evolution doesn't run on "need."

Some baby Gorn will stumble to maturity, and those individuals should be better at killing, while being no less capable of reproducing. No flood of fresh-out-the-eggsac lizard mantises is about to outsmart the killiest Gorn of her generation.

Selection pressure is thus toward Gorn who can survive their own adolescence, or at least survive long enough to reproduce. Adult Gorn are a goal, if not the goal, even if getting there takes a few recursions. The genes want to be inside a cannibal with a spaceship.