I'm getting the error Sentinel ... not found

You've most likely disabled creating debug symbols in your cargo bench profile. This can originate in an option you've added to the release profile since the bench profile inherits the release profile. For example, if you've added strip = true to your release profile which is perfectly fine, you need to disable this option in your bench profile to be able to run Gungraun benchmarks.

See also the Debug Symbols section in Installation/Prerequisites.