What benchmark would you recommend for testing a gpu?
There are all kinds of good benchmarks. Unigine makes Valley and Heaven which are both good. You could run FurMark to stress the GPU to make sure your temps are good. 3DMark has many benchmarks, the latest being 3DMark (oddly enough) which has Time Spy a DX12 benchmark.
Oh, also forgot the Cinebench R15 has a GPU benchmark for OpenGL.