G-hash is Giga hash, and a One gigahash processes one BILLION hashes per second. You need a high amount of this to successfully mine crypto coins. Gigahashes are not just dedicated to bitcoins but are calculated in all the algorithms actually. Its just SHA coins (bitcoins) use a lot more hash than most other algorithms so it is more often refered to when mine taling about BTC however doge and litcoins are stepping up to that plate quite quickly..
