Trainer GUI runs on Directx so yes it uses some GPU but usually it's literally not noticeably (less than 1%). 40% sounds like a bug, my first thoughts are:
1. Something hijacks into the DirectX drawing process and tries to run an overlay onto it. A very popular candidate is MSI Afterburner but anything else that uses an overlay can cause it as well.
2. A resource isn't free'd up upon use and causes a high GPU load on long run. You can easily verify this if you run the trainer and it isn't using 40% GPU immediately but only after some hours.
3. Unlikely but maybe there is some deeper Microsoft / Graphic card problem that causes such a problem on a specific graphic card with a specific driver.
I think something in the fall update borked something with Gsync displays, DX programs, and maybe the nvidia drivers. I've noticed seriously increased usage with programs eating GPU power when they shouldn't have done so. I had to keep the AC Valhala trainer minimized to avoid this. It's like it's trying to apply windowed gsync to the trainer, and flickers itself to death when it can't. It doesn't seem like it's the CH trainers more than something broke in windows last fall.
I figured it might just be me, since most people aren't on the 3090s with 4k120Hz gsync panels, which is niche, and niche usually has issues.
TL;DR seems like windows has issues with nvidia lately.