"Gay Cum Swallow" is a porn video tag that specifically refers to homosexual male sexual activity. In this context, "Gay" indicates that the participants are men who have sexual relations with other men. "Cum" is short for "cumming," which means ejaculation during orgasm. "Swallow" in this context implies that one partner (usually the bottom) allows the other partner's (top) semen to remain inside their mouth and throat rather than being expelled through a reflex called gag reflex. This act is usually performed as an expression of love, intimacy, or submission.