The mining process used within the GFS app requires a minimum of 4GB of VRAM to be able to work with AMD cards. Please know that even with 4GB of VRAM, some users report an inability to mine, and this is something we hope to resolve in a future Beta. For now, AMD cards with less than 4GB of VRAM very likely will not work.
If you have more than 4GB of VRAM, please make sure you have current drivers and that your GPU isn't on the list of unsupported GPU's (see What graphics cards (GPUs) are / are not supported?)
This 4GB VRAM requirement is due to the current DAG size of Ethereum.
DAG is a Directed Acyclic Graph. It is used in all Ethash coins, like Ethereum, Ethereum Classic, Musiccoin, Expanse, Ubiq and other coins to provide a proof of work.
Read more: What is a DAG file? by Investoon