When I buy a computer, besides the price, one of the most important characteristics is the processor. CPU DB is a website that aims to list cpus hardware specifications.
Where can I get similar data for GPUs (for multiple brands)?
TechPowerUp is a not ideal, but reasonable answer.
It is not downloadable (at least not in a well structured format), but it provides a database with Chip name|Release date|Bus type|Memory|clock frequency|Memory clock|Shaders/TMUs/ROPs information about GPUS. It is possible to select data concerning the all the provided fields as well as the generation.