In principle this is not necessary: By default, GPC understands all of these dialects (as far as implemented) and you can specify one (or more) of `--standard-pascal', `--extended-pascal', `--borland-pascal' to restrict GPC's language to the specified dialects.
This is good. But where they conflict, e.g. the results of write('abcdefg':3), I expect that they would follow standard-pascal or extended-pascal, not borland-pascal.