All there on the GPS site like HydroRed linked. Plus you can earn 10 nuggets just for checking into the site everyday.
So the basic math is 1 nugget equals 1 cent ($0.01), since 100 gold nuggets are worth $1.00 USD (as stated on the website). Every dollar spent on gear from GPS gets you this return:
Bronze tier $0.02 for every $1.00
Silver tier $0.04 for every $1.00
Gold tier $0.08 for every $1.00
Diamond tier $0.20 for every $1.00
Everyone used to get $0.50 for every $1.00 spent, so the drop off is significant. I'm not trying to talk down on GPS or Gu at all, just stating facts. I don't think anyone else in the game has any reward system like Gu, so I am thankful for it, and I'm sure it has benefited both GPS and the consumers. I have way more packs from GPS than I really need because of gold nuggets. I don't think I ever paid more than $25 a pack except for the one Copper Chem for $35 w/ nuggets applied. So I'm a happy man. Cheers!