From September 22, 2017 to September 21, 2018 if you buy one of the VC releases of either Pokemon Gold or Silver, you'll get a one-use serial code to redeem it on your Gen VII game.
In addition, you'll also get a 3DS Home Menu theme of your version if you make the purchase before January 15th in North America, December 31st 2017 in Europe or September 28th 2018 in Japan.
Sources: English official site Japanese official site Korean official site