Is there any way to find out and/or limit GPU usage by process in Windows?
The answer to your subject line question is: yes (On Windows Vista and up), use Process Explorer from Microsoft to monitor per process GPU usage. nvidia's parallel nsight can do this also. Now, the body of your question sounds like you want to do this remotely. Unfortunately I'm not aware of a way to do this remotely. Still, hopefully this will be of some use to you.
edit to add: If you fire up Process Explorer I don't think it shows the GPU stats by default, to get them right click on the list of columns and add them.
The GPU is a resource that can only be used by one program at a time. If another process is using the GPU, then you can't get access to it.
A program may run multiple GPU kernels at the same time, but it's up to that program how those get run. There's no real concept of scheduling like there is with the OS and CPU processes.
Some vendors may have a way for you to check on the status of the device, like # cores in use, heat, fan speed, etc, but that won't let you change what's happening on it, and it will be specific to each vendor/device.