CAUTION: Spoiler Ahead--Out of the Abyss, hard cover
In a recent game, mentioned above, the party had all their NPC's banished to the NPC's home planes, including a couple of pet Death Slaads. At some point thereafter, the party time-traveled back in time, which was a possible outcome in the campaign as written.
I'd like to use a Gate spell, or something similar, to retrieve my pet using the control gem I still possess, but the DM claims this is impossible because when we time traveled, the pets were left on a time-line which is no longer accessible from the time-line we are currently on. I've asked him to explain RAW how his parallel time-line concept (he admits this is his own creation) makes a plane inaccessible via a 9th level spell, which is RAW specifically designed for planar travel/portal creation. So far he hasn't.
By the way, this is an Adventurer's League game where the rules more clearly define the limits of play, not a homebrew where the DM can willy-nilly justify anything he feels like.
The question then, is whether there is any RAW justification for invalidating a Gate spell, similar magic, or another means of planar travel and retrieval, on a basis of a time-line distinction?