nope, Gab was not immortal.He probably was. He just got separated from her before that could really be seen. Companions have too short of an effective life span to actually test their natural one. Only ones who make it to old age are the ones who never fully awaken their connection and there's no logical reason to expect them to benefit from any potential longevity increase. Nael never cared enough about humans to take a companion before Gab so Amy and MC are the first chance to really test that thought. Sam would quite likely have no way of definitively knowing those limits.
Sam also doesn't seem to appreciate the actual limits of Nael's power yet. She's managed to bypass his restrictions quite a few times and also managed to hide her godhood from him until Gab made her snitch herself out. She's still interfering with the world even now which seemed to be against Sam's rules for her.
and Sam did know. he confronted Nael after she revealed her godhood. Sam is implied to be a god that rules over life, he would know. he said that Nael could prolong Gab's life for a long while but eventually he would die. and no, Nael cannot interfere directly anymore, except for the portal. it might have been allowed so she could still see Gab and avoid her being frustrated and upset too quickly. though, Gab dies killed by Dia's companion, Nael destroys the universe. so "not allowed to interfere" might be just because the other gods would punish her or something like that.