[mchannon]

GPU's aren't used for mining Bitcoin. Bitcoin's algorithm is CPU-intensive, and is nearly exclusively done on from-the-Silicon-up purpose-built chips like Avalon and Antminer.

GPU's are used for mining other cryptocurrencies like Litecoin and Ethereum. These can be traded for Bitcoin or fiat currencies.

[simias]

Bitcoin was GPU mined for a while (it's "just" SHA-256, it's not particularly designed to be GPU-hard unlike other PoW algorithms) but you're right that since ASIC mining became the norm it's not really cost-effective to mine BTC on the GPU.

Some other cryptocurrencies have algorithms that are designed to be hard for GPUs and ASICs (cryptonote for instance) but I think it's just postponing the inevitable. If those altcoins become valuable enough I'm sure people will start making micro-optimized dedicated hardware to mine them and eventually it won't make sense to use GPUs for them either.