To request a GPU slot for a job, you can simply add the attribute request_gpus (as well as request_cpus and request_memory) to the job.
Currently, a job can only use 1 GPU at the time.
request_gpus = 1
request_cpus = 1
request_memory = 2 GB
Note the number of GPU resources in CMS is still limited at present, so the matching can take longer than regular (cpu) jobs.
It is currently not possible to specify exactly what type of GPU you want, but you can match on for example CUDA compute capability. For example, use the following requirements expression in your job:
requirements = CUDACapability >= 3
You can find an example on how to use GPUs with TensorFlow workflows here.