Without being there it's hard for anyone to know what's going on. What you say attack might be his way of playing. Grabbing him and playing tug with the labs leg could have escalated it. He very well could have been attacking him, but there would have been serious damage if if was a full blown attack. I wouldn't risk the labs safety, but I wouldn't call the Gsd dog aggressive because he might not like one dog, but he likes the other three. He might not like other males.
I agree with you llombardo. He might just not like other males if he's fine with females. If he was dog aggressive, he'd have an issue with other members of your pack. OP I'm sorry your going through this. It can be pretty scary when there's conflict, especially when fostering with a new breed.
Personally I have fostered two shepherds...one female, one male. My pack consists of a female lab and a male rescue german shepherd. My male is very selective with male dogs and doesn't like when they try to play with my lab. Our first foster was a senior girl, low energy, very sweet not dominant at all. He was indifferent to her. He never really warmed up to her, just ignored her. We ended up adopting the female. Sadly she passed from cancer not long after. I don't think he missed her one way or another.
Now fast forward to now. We have our puppy who is a male german shepherd, along with our lab and male rescue boy. We are fostering a male german shepherd. Prior to bringing him home, I tested the foster out to see if he'd get along with another dog. He is very thin and shy. Not dominant at all. Very sweet temperament.When I brought him home, I introduced him to my male rescue first. Then one by one all the others individually. Not sure why, but my rescue who is very dog selective LOVES him. In the month and half we had our senior female fosture failure before we lost her...he only started to pay her attention the last two weeks. It took him three days to accept Balto (possible foster failure). They play together, lay together and are best of buddies.
With your foster, it takes time. He's in a new environment and your not sure what his past was. If he is fine with you females, but not your males, it might be he's dog selective. If he is dominant and you other male is to... two just might not mesh together.
Are you familiar with your foster's story? Maybe that can tell you a bit about him. Before you fostered him, did you all check his reactions to male and female dogs since thats what you have. It might be something with just your male or it might be all males...but would be goo to know for him down the road. Is he your first foster? The thing with fosters is you really have to understand the dynamics of your own pack and understand the temperament of the foster dog. You want the experience to be positive for all, especially the foster. I wish you luck : )