Gift cards to your local FLGS or to the OneBookShelf family of sites (DriveThruRpg, RpgNow, DmsGuild, etc.) or to Amazon. Getting specific books for your GM is always tricky - they may already have it or someone else might also be getting it for them for Christmas. A gift card allows them to buy what they think they need, which is always nice.
And if your GM also hosts your games at their home, food and alcohol always make great gifts at the holidays.