I personally find Gangarel having a son to be likely but siding with the shepards after being under Validar's tutorlage for so long to be unlikely. I'd expect that sort of person to go out on their own and be a force opposing Lucina and Grima. There's also the question is how did he survive like Lucina did. Her's is questionable in it's own right but Plegia is the home of Grima worshipers. It'd make more sense if he was the bastard of Walhart or some other Valm lord.