Once your vet checks him out (and presumably, nothing's wrong), stop worrying.
When Campeche was a puppy two years ago, everyone that year (it happens every year) was mentioning what their dogs weigh. Camper was on the low end. Dh made me a spreadsheet so I could track Camper's growth. It was super consistent, not really spurty at all, and that made me happy. My vet was pleased with his health (despite ALL his allergies), and I kept telling myself that he was Fine. Just Fine.
(I also wasn't feeding puppy food. So I kept telling myself that too. And he's always been ridiculously active).
Camper is now 85 lbs and 25.5" at withers. He's doing that whole filling-out thing that happens between 24 and 36 months, so he may pick up another couple pounds. Or not.
But my smallish puppy is a perfectly *standard*, rock solid GSD. He never got as big as his sire. But I'm good with that. When I have to lift him occasionally, I'm glad he's not bigger and heavier.
If your vet thinks Gunner is too small, he likely will have suggestions, starting with maybe bumping up his caloric intake a bit. But talk to your vet first. Some pups weigh a lot because they're growing too fast (due to food usually), some because they're actually carrying some extra fat, but most because they have genes that tell them to grow fast early but slow down later.
And most "little" pups grow up to be just as magnificent as the other guys... Well, I think more so!
But I'm horribly biased.