Gary, who preferred the nickname "Gaz", was a young human boy who was holidaying in Blackpool in 1976 when the S'Qwatch invaded. He accompanied the Twelfth Doctor - who took to calling him "Gaz-better" after he told him that he "lik[ed] Gaz better [than his real name]" - while he sent a signal for the S'Qwatch's mother to come and collect them. (COMIC: A Cold Snap)
If a medium is not mentioned, then this incarnation did not have companions who were original to that medium; it does not mean that this Doctor failed to appear in that medium.
