there is really no simple solution it always depends on the dogs involved. the first thing i think i would try is talking to the irresponsible owner who let's his dog run about willy nilly. if that doesn't work i would think about another route. if that is not possible there are a couple of last ditch methods you can try. remember the important thing is that the dogs don't make eye contact or posture etc or else it will escalate with lightning speed and then things get ugly!

1) i think your best bet is change directions and walk away (keep your own dogs focused on you -- use treats if you have to -- to keep your dogs eyes on you) that way the gsd perceives he has done his job and scared you off!

2) you could put yourself between the oncoming dog and your guys this way you can prevent direct eye contact and it will confuse the oncoming dog a bit -- if he has to think then you now have a few seconds to react with your dogs and go another way or the other owner can call his dog.

3) instead of yelling no put your arms straight out in front of you (typical stop signal) and say sit in a stern voice. sometimes it works and then the owner can get his dog under control.

4) keep a load of treats with you if the dog is still coming toward you and nothing has worked throw them at the dog and aim right for his nose (more likely to smell them that way) when they scatter hopefully he will go after them again buying a precious few seconds to react and get your dogs out of dodge!

just a few suggestions i hope they help