I'm just curious.

think it through:
i suppose disk encryption does affect hard drive performance and adds stress on the CPU, if not much.
since GPUs don't typically access the hard drive, i'd say it isn't affected by that, not directly. except when the driver is loaded form disk at startup maybe.

Kind of an interesting question.

More and more encryption is being performed by gpu's. I kind of doubt it is currently being used to work on disk encryption in your case but there are plenty of examples on how to use gpu for different encryption tasks and for disk encryption tasks.

I'd agree that in real world examples you wouldn't find much overhead being consumed by the GPU. More likely that other bottlenecks could present some degree of speed loss. There is real overhead by using almost any encryption from disk to wifi so a good set of performance reading could decide.

i didn't think of that. but surely the linux kernel doesn't do that for standard disk encryption?

I don't think it does on any mainstream distro yet. Or at least I don't know of it. I could be wrong.