You can't trade your coal for gold. Yeah you can use the game you won on Steam Gifts and make a giveaway on GameMiner as long as the game is in their database.
1. In steam gifts you accumulate points, and it seams coal is the equivalent so how do you enter giveaways that needs 15 Gold , what do you do with coal?
2. Isn't that against steam gifts rules? if I am not wrong you can't use what you win on steam gifts to make a giveaway, I'd say that include other sites too?