If the weather is very warm and they have a place to get dry if they want to, I would leave 6 week old goslings out in a light rain. A heavy downpour I generally brought them in (if I'm raising them myself). I've noticed the parent-raised ones being treated the same. It's always pretty warm here by the time goslings hatch, so I can't comment on what parent geese do in cooler weather.
 
If it were me, I'd want to bring them in if it were cool though, since the down feathers don't shed water well and they can get really soaked. I'd probably factor in wind too ... if it's very windy I'd be more likely to bring them in.
 
BTW, I'd say the same for swimming. I only allow swimming for brief periods in warm weather before they get their feathers in, and you have to watch that they don't drown. Same for ducklings. But once they are fully feathered (a few weeks older than yours), I don't do anything special to protect them from being wet. Again, I've always raised them when it's already warm.
 
Geese are pretty tough though, and don't need to be protected NEARLY as much as baby chicks. I just don't like to see them wet to the skin for more than a brief swim (maybe 30 minutes).