What do you use gold bolts to buy?

Gold Bolts are used to buy Gold Weapons which are found later on in the game. Your first chance to purchase Gold Weapons is on Gemlik Base, Oltanis Orbit after you defeat Qwark. On the runway where you took off your ship, look for a tower. If you Wall Jump between the two walls of the tower, you will find a lift (elevator) if you press the switch on that, you'll get the first five Gold Weapons. (Gold Bomb Glove, Gold Pyrocitor, Gold Blaster, Gold Suck Cannon and either the Gold Glove Of Doom or Devestator)

The only other chance is sfter you beat Drek AND restart a new game. Once you get to Eudora, there's a teleporter up by the Gadgetron Vendor which will give you the option to buy the other five, if you didn't buy any of them from Gemlik Base, they will still be for sale here,

Gold Weapons

Gold weapons.

Go to the second Space base when you stert you need to turn around and walk to the portal. Go trough it there will be a bridge to your left, cross it and go to the tower right in front of you. Walk to the tunel trough it and right on the edge of it and I meen right on the edge like the first 2 or3 feet of it. Wall jump up then you get to a ledge jump up to it and ride the elevator up. Theres only 5 out of the 10 gold wepons. 4 gold bots and 20,000 normal bolts form each gun at this gold gun.

To buy Gold weapons at Novalis after completing the game or at Oltanis space station (see youtube on how to get to gold weapon room which is simple)