The GNU back end (gbe) is used to convert RTL into assembler code. It is supposed to be language independent. Files are in the .. directory (i.e., the directory called gcc). It also uses files in the ../config subdirectories etc.
Unfortunately, some of them are not completely language independent and need patching for GPC. These patches (against all supported GCC versions) are in the diffs subdirectory.
The Pascal language implementation files are in the directory called p. Some of them were written from scratch. Others are hacked from GCC sources. Their roots, if any, are mentioned in the comment at their top.