I understand there are external tools to do so, but is they a quick key or .exe launch property that can be used to have the FPS / GPU usage in the game?
You can also use nVidia's "GeForce Experience" program for that. I do not consider that as an external tool, I think in most cases it's already installed and running on your system if you have an nVidia GPU.
XBOX game bar included in Win 10 does the job, even shows CPU/GPU/RAM/VRAM usage. For some odd reason, you need to provide admin rights to show fps, which does not really work with the usual prompt you get in Windows. Even if you confirm to allow admin access, it will not work this way. But there is a permanent workaround. You need to add your user to the group "Performance Log Users" Here's how: Link It's in Spanish, but translated to English underneath.