Wow that's super cheap! Yes it will work fine in a Golf, mines actually originally from a Leon.
You shouldn't need a new gateway either as no one has ever reported the RNS 310 specifically as causing drain, its only the RCD 310, RCD 510 and RNS 510 that seem to cause problems. I'm still using an ancient version F gateway and have left my car parked for well over a week without any drain whatsoever