If someone’s ‘havin a gaff’, I’d assume they’re quite young? If it’s ‘an empty’ bring booze for you and you alone. If it’s a more mature affair with dinner, I’d bring a cake for dessert (for everyone). If it’s just a bunch of adults gettin on it, they won’t expect anything but maybe find out what the hosts like to drink and bring a bottle of that for their stash if you really want to give them something.