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.