Is "gaf" a valid word?

Yes, gaf is a valid word depending on spelling, context, and dictionary, though its validity differs between standard English and slang. In standard Scrabble and most English dictionaries, gaf is not considered a standard English word on its own, but gaff is a valid word. Gaff is a noun with several meanings including a hook or spear used for landing large fish, a metal spur attached to a gamecock, a cheap theatre or music hall, and in slang meaning a house or home in UK and Irish English. Gaff is valid in Scrabble and is in Oxford English Dictionary. The spelling gaf without the double f is commonly used in informal texting and internet slang as a shortened form of gaff meaning house, particularly in UK, Irish, and Australian slang, such as my gaf meaning my house. In that slang context, gaf is valid as slang though not formal standard English. Additionally, GAF in uppercase is a valid acronym and proper noun for companies like GAF Materials Corporation, the roofing manufacturer, and is recognized as a trademark. Gaf is also a valid word in other languages, including Dutch and Afrikaans where gaf means gave. In internet slang, gaf is valid as shorthand for give a fuck in phrases like idgaf. So while gaf is not a formal standard English word in most dictionaries, gaff is, and gaf is valid as slang, acronym, and in other languages, and is widely understood in informal British and Irish English to mean house.