According to Laussy Fabrice:

Is the way to tackle a part of code directly in code machine with GPC is to use RTL?

No: You write inline assembler code in the AT&T syntax.

Yes: The interface between the assembler code and the Pascal program has a lot common with RTL.

For documentation how to use assembler in GPC, see:

ftp://agnes.dida.physik.uni-essen.de/gnu-pascal/contrib/gpcasm.zip

Good luck,

Peter