CX1 contains a small number of GTX Titan, K80 and P100 GPUs. These can be accessed by adding ngpus to the #PBS resource request. For example:

#PBS -lselect=1:ncpus=1:mem=8gb:ngpus=1

PBS will assign your job whichever model of GPU first becomes eligible to run your job. Restrict jobs to a specific model of GPU by adding 
gpu_type to the resource request. For example:

#PBS -lselect=1:ncpus=1:mem=8gb:ngpus=1:gpu_type=GTXTITAN
#PBS -lselect=1:ncpus=1:mem=8gb:ngpus=1:gpu_type=K80
#PBS -lselect=1:ncpus=1:mem=8gb:ngpus=1:gpu_type=P100

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.