The game was originally announced back in November during BlizzCon 2014. Unlike the Curse of Naxxramas expansion, Goblins vs Gnomes will be a much bigger expansion. The expansion will see the addition of more than 120 new cards to the game which will include new mechanics, minion types, gameplay modes, and so on.
The expansion will be free for download which is unsurprising given that Hearthstone is essentially free. However gamers who wish to get additional cards will have to pay for it by purchasing booster packs. Either that or they can farm for gold and/or craft the cards themselves by farming for materials.
The game is currently available on the PC, Mac, and the iPad. Android tablet support is expected to arrive by the end of the year, as mentioned previously by Blizzard. However support for the iPhone and Android smartphones will be coming in 2015.