Photon Geyser deals damage. If the user's Attack stat is higher than its Special Attack stat, Photon Geyser becomes a physical move; otherwise (i.e. if the user's Special Attack is greater than or equal to its Attack) it is a special move. During the execution of Photon Geyser, all ignorable Abilities that could affect the success or damage of a move are ignored, except when Photon Geyser is called by another move rather than directly used.
For determining which stat is higher, stat stage-modifiers are taken into account, but other effects (e.g. held items such as a Choice Band, Abilities such as Huge Power, burn, etc.) are not.
Photon Geyser does not exist in the game data of Pokémon Sun and Moon. As a result, a Pokémon knowing Photon Geyser cannot be traded from Pokémon Ultra Sun and Ultra Moon to Sun and Moon, and if a Pokémon in Pokémon Bank knows the move, it cannot be withdrawn in Pokémon Sun and Moon. In order to be transferred to Pokémon Sun and Moon, the Pokémon must forget Photon Geyser.