In this porn video tag, "gay" refers to the sexual orientation between the participants. It implies that the sexual activity depicted in the video involves two males, or one male and one transgender individual with male parts.
"Deepthroat" is a specific oral sex technique performed in the video. The giver (usually a male) performs fellatio on the recipient (also usually a male), taking his partner's penis deep into their throat, down to the base or as far as they can comfortably manage. This tag indicates that the video contains explicit content of this oral sex act between two males.