PBS will assign your job whichever model of GPU first becomes eligible to run your job.
Within the context of the running job, the shell environment variable CUDA_VISIBLE_DEVICES will be set with indices of the allocated GPUs. Jobs must respect this setting, or they will interfere with other jobs co-located on the execution node.
Only do this if you must have a certain type - your jobs are likely to spend much longer queuing as a result.