It was earlier that GPU systems were used to mine bitcoins.But now,mining difficulty has increased a lot and so,bitcoins could only be mined with powerful ASIC miners.China is the leader in manufacturing ASIC miners and only for this reason,largest number of mining farms are located in china.

If you just want to mine altcoins,then even GPU systems could be used to mine them.You could ust download minergate software and star mining.But it may even damage the motherboard of your system.

It has been said tht mining logarithm of BTG hs been changed so that even GPU system could be use to mine Bitcoin gold.But,till now,no official statements has been made clearly.

But still,mining altcoins using GPU systems seems less profitable.