There are several synonyms for the phrase "hold a gun to head." One common expression is "put a gun to someone's head," which has a similar meaning of threatening someone with a firearm. "Hold a weapon to head" is another option that is slightly more general, as it doesn't specify a gun specifically. Other related phrases include "point a gun at someone," "aim a weapon at someone," and "brandish a firearm." Each of these phrases conveys the idea of a serious threat, and should be used carefully and judiciously in both spoken and written language.