From what I can understand, Gigantamax Machamp and Gengar aren't entirely exclusive to specific games. According to Bulbapedia, in normal gameplay GMax Machamp and Gengar are exclusive to Sword and Shield, respectively.
However, these GMax forms have been made available to BOTH games through Wild Area News. Additionally, through the purchase of an expansion pass, Gengar and Machamp can be made available to both games.
(sorry if this isn't good, it's my first answer)