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