There are countless digital currencies in existence; every day new ones appear, are pre-released; ICO'd, or are ninja launched.
Bitcoin is SHA-256, which renders mining using CPUs and GPUs pointless because ASIC processors have been engineered; chips which serve only one purpose; that is to compute/solve/hash SHA-256, and because they are engineered to do only that; they are exponentially more efficient in doing so.
GPUs on the other hand are used to mine coins both by hobbyist miners and large scale GPU farms.
But they aren't mining Bitcoin; or any other SHA-256; they are mining coins either with new POW (Proof of Work) algorithms; or POW algorithms for which no ASICs have yet to be engineered (or are simply not public/available).
For GPU based setups; common algos are X11, X13, X15, Scrypt depending (as ASICs do exist for Scrypt but were not widely adopted); and many others.
You can find a list here: http://bitcoinguide.online/a-list-of-cryptocurrency-algorithms/
If you only have a single GPU; there is really no point whatsoever in using it to mine a currency; unless it's your own clone that you're creating.
You can go on site's like NiceHash and rent mega power; based on bidding per / g/hash t/hash; and jump in and speculate on what coin may or may not be profitable to mine. One GPU simply wont cut it.
Or you could build GPU mining rigs; using a simple steel frame; a low end CPU processor; 4 GB of ram; run Ubuntu off a USB stick; and slap 4 or 5 GPUs on the motherboard using PCI-E Risers. You'll need a decent 1300w or more PSU; depending on the algo; and you must be network and code savvy; as it's mostly all still command line driven.
If you do so; expect to lose money the first year; it's still the wild west; and the only way to learn it is in the trenches.
And to add; yes; AMD shares have been volatile due to alt-coin news and releases. As have prices. Cards, such as an AMD R9 280x; one of the most popular first gen cards at the time Alt Coins came crashing onto the scene; skyrocketed in price from $200 to $500 or more on all sites; including Amazon. As did all components; PSUs, etc.
Then; they would crash with the market; people would burn out; the secondary market became flooded with people ripping their rigs apart and selling off the pieces; and then repeat-- new coin comes out; it becomes a huge rally, and the card prices would jump back up... sometimes doubling, tripling in price in days... People made money of that as well; simply buying GPUs on the crashes and selling on the rally's.
I often joked that Bitcoin; and later the alt-coin clones which appeared in hoards were invented either by the Power Companies; or the Chip/Wafer manufacturers.