Is #pragma once a safe include guard?

I’ve read that there is some compiler optimization when using #pragma once which can result in faster compilation. I recognize that is non-standard, and thus could pose a cross-platform compatibility issue.

Is this something that is supported by most modern compilers on non-windows platforms (gcc)?

I want to avoid platform compilation issues, but also want to avoid the extra work of fallback guards:

#pragma once
#ifndef HEADER_H
#define HEADER_H

...

#endif // HEADER_H

Should I be concerned? Should I expend any further mental energy on this?