I am working in the automotive business, and they have a lot of embedded systems these days. Not that I have seen more than a couple of them, but none had GCC.
Do the compilers for those platforms have decent support of all C++ features? Memory allocation? The functions in <cmath>? Templates? Exceptions? If the answer to all of those is "yes", then you have a point about warning about lack of support for `#pragma once'. If not, having to use header guards is one more oddity about this environment that you have to be aware of, and the OP should feel free to use `#pragma once' in other environments.