I dunno. I know there's lots of appeal in the kid looking like either Gary or Ash, but I think bringing in other traits from their families would be just as plausible. For example, my niece, born last year, had red hair. Her parents are both dirty blond, so the hair was likely from my sister's mother and her family or my dad's mother and HER family. It's quite possible that as long as Gary was involved, they'd end up with a child similar to Sammy (young Samuel Oak), or if it was Ash, someone looking similar to his mother. We also have no idea what Ash's father looked like, as well as Gary's parents, (if you don't go by their brief appearance in the manga), so they could end up with children with THOSE looks as well. I like to imagine Gary's mother was blond, really, and while it's possible Ash's father passed on all his physical features to Ash, we probably won't ever know for sure.
tl;dr: There are more traits available for a child for Ash and Gary then the ones either boy already has.
EDIT: If you need an example of this from the show, consider the fact that Misty and her sisters ALL have different hair colors. Assuming none of them dyed it, there's no way their parents had all those colors on their own heads: at least two colors would have had to come from someone else in the family line.