There's a new-ish system Blizzard added to WoW (unsure about other games) where you can exchange gold for WoW Time Tokens. It was their compromise for "Free-to-Play," while removing copious amounts of gold from the in-game economy. Thing is, the market is somewhat regulated by Blizzard but at the end of the day the playerbase has control of the prices. Long story short, you exchange your gold at the auction house for a token at whatever price it is currently available at, and said tokens can be changed into either game time for WoW (1 month) or the equivalent ($15) in Battle.net cash, which allows you to purchase things from the Battle.net shop, including other games. Destiny 2 will be launched on Battle.net, since it's Activision's launcher, of sorts, and bungie and Blizzard are buddies. So yea, you can use the money from exchanging your gold to buy Destiny 2.