I have a gsp pup as well (he's almost 5 months) and I fed him the recommended amount on the back of the container, in this case it was 1.5-2 cups 3 times a day for the first 3 months. I just went with that and didn't have any problems. as they get older you will need to feed more. I'm now feeding about 3 cups 3 times a day. I think I got lucky with my pup, he only eats when he's hungy no matter how much or little I put in front of him. he's not fat or skinny, just in between where he should be.

-Scott