Yes, they can all use the same GPU. By default, anything you run on the same GPU from different processes will run in serial. However, if you are also doing a lot of host-side code, the other workers can be getting on with that while they take turns with the GPU, so you can still get a benefit. Just be wary of how much memory you are using. By default, each MATLAB process will hog up to a quarter of GPU memory. If you have four or more workers and you're using a lot of memory, you could find your GPU running out.
If you are on Linux, you can run the NVIDIA Multi Process Service to allow each process to use the GPU concurrently. However, this often doesn't gain you much, because code that is using the GPU 'well' will not have any spare compute for another process. A bit like multi-threading on a single core CPU, the apparent concurrency is still bottlenecked by the fact that there's actually only one processor.
