The Pope receives that gold (--> always vote Yes when you're the pope, always vote No when you're not, is the safe bet).
The amount is variable but just a meager windfall: Only choose this option when you can't benefit from your own Papacy otherwise. The Tithe is the resolution most likely to succeed: Most of the time you can't get your catholic subjects to agree to a crusade against <your enemy>, but most times everyone agrees to a Tithe.
I doubt that the Tithe is deducted from everyone, and I think it is generated out of thin air but dependant on the number of catholic cities (2 gold per city or possibly 1 gold per catholic inhabitants... or something?). May be wrong in this regard however.
