Don't have a G3...But if the valve is original and your amp is a bit dated...Changing that can only help...I know nothing of the G3 so I'm assuming it must have a 12AX7 tube or two in the preamp...If that's the case, those tubes are easy to get and cheap...Tube Depot, whomever can get you a winner for 20 bucks or less including shipping...
Edited to add: Oops, just read the specs on your amp and it uses a 12AT7 not 12AX7...If you wanted more gain, I suppose you could try a cheap 12AX7 although I don't know how that will overdrive the MOSFET circuit...Nonetheless, buy a new tube or two and experiment...For my money, standard Tung Sols are great although JJs seem to be most popular/standard in Randall gear...
Good luck!