When I unlock achievements on Xbox LIVE, I get rewarded with Gold. Here is an example of Gold giving achievements in Wordament worth 10-20G:
Collectively, from different games, they accumulate on my profile:
Where can the Gold be used?
G just means its a gamerscore. Gamerscore is more for bragging rights. Xbox just implemented a rewards program http://rewards.xbox.com/.
If you have a certain amount of gamerscore you are put into three categories which can be found here: http://rewards.xbox.com/myachievements/
Contender - 3000-9999 gamescore and you get this is a reward:
Champion - 10000-24999 gamescore and you get this is a reward:
Legend - 25000+ gamescore and you get this is a reward:
I think the gold which you are referring to (example: "swipe those tiles 10G") is in fact something called GamerScore (abbreviated G), you can't really use it for anything besides showing your friends and competitors how accomplished you are at a game or games. I do know you get some perks on your b-day but that's about as much as i know. On top of that you can't spend it anywhere, if you had MSP (Microsoft Points) then you could use that at the XBOX Live Marketplace where you can buy games, avatar items, etc.