I don't have the save where barbarians had it but it was after turn 20 and when I captured it nothing special happened, I just gained control of it.
I reloaded the save where I spawned 6 turns away from the Guardian and tested a bit more. I kept the fort commander there until turn 21 and that didn't cause anything to spawn. I then deleted the commander and moved my scout next to the Guardian and the Gargoyles spawned on the Guardian while it was still under my control.
With this in mind it may be possible that in the save where I thought barbarians gaining control of the Guardian had caused the Gargoyles not to spawn, the Gargoyles had been spawned and killed somehow. I don't know how that would've happened as the only nearby civ was Lanun with archers as their strongest units and the triforce promotions were nowhere to be seen.
I reloaded the save where I spawned 6 turns away from the Guardian and tested a bit more. I kept the fort commander there until turn 21 and that didn't cause anything to spawn. I then deleted the commander and moved my scout next to the Guardian and the Gargoyles spawned on the Guardian while it was still under my control.
With this in mind it may be possible that in the save where I thought barbarians gaining control of the Guardian had caused the Gargoyles not to spawn, the Gargoyles had been spawned and killed somehow. I don't know how that would've happened as the only nearby civ was Lanun with archers as their strongest units and the triforce promotions were nowhere to be seen.