In SNW 1x09 All Those Who Wander, the crew reenact Aliens with a handful of baby Gorn as their adversaries. We learn that Gorn breed by infecting a host animal with eggs, which hatch and burst out of the host when mature (which can take months or hours, apparently depending on the host). The babies are immediately hostile to other baby Gorn, and are left to their own devices until they are picked up by adults at some indeterminate point. We also learn that these baby Gorn are themselves capable of implanting eggs in a host by spitting on them.
These baby Gorn seem like a full fledged viable species already: small, vicious hunters who are (like tribbles) basically born pregnant. From an evolutionary perspective, that's plenty to propagate their own existence. It's also a lifestyle that selects for intelligence (small hunters tend to be pretty smart) but seems like an unlikely route to developing genuine sapience. We'd expect these baby Gorn to have a relatively stable population given the turnaround times of egg maturation and their predilection towards cannibalism, and the later feature would also make it far less likely that any given individual would survive long enough to become an adult, as each fresh generation brings a wave of fresh adversaries who vastly outnumber the handful of survivors from previous waves.
Of course, we know there are adult Gorn. So, how did they come to be? 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?
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.