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.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD