Is this a pure breed GSD?

Could be.  Weight?  Age? 

I am not sure. I found it homeless and neglected.

Can you get a couple more pictures? Especially from the side, with the camera pretty much on the dog's level?  

I'd like to see more pictures, too, but it looks like one to me.  

It's hard to take good photos of a little puppy with my camera phone, but here is another.

Hard to say without a weight/age comparison.  It could be a GSD, but it could be a hound mix or a number of other things... It looks to have a REALLY tight coat.  Even short stock coated GSDs can be pretty fuzzy pups. 

The coat is really tight, the tale is not fuzzy at all. All I know is that he is teething (that's the best I can say about age) ...

That could indicate part (or all) GSD then, typically their ears drop while teething. 

 After seeing the pic and hearing he's teething, I'm going to say no. Because a 4-5 month old GSD is going to be larger than this pup looks. Also, his body type doesn't say GSD to me. As far as what he is, I agree there may be some hound. He's a cutie.

In the second pic, it looks pretty leggy in the rear, lacks angulation, no deep GSD chest.  The coat does seem really short/tight for a GSD, but I've never had a GSD puppy myself.  The face looks GSD-puppy, but it's hard to say.  The ears could be down for teething, or just be that way.  It could easily be some mix with lab or hound.

Maybe it's just me, but the pup's belly looks a bit swollen.  Has it been wormed? 

Yeah he is a real cutie. Very active. Good to see him put on some weight.

growing up we owned many shepherdxdobermans.... he looks likes he could be a mix of that. sadly i dont have any pictures online of the dogs i had when i was a kid... and yes the teething stage for those guys was the same size range as this one.

I am just about to worm it and take it to the vet for all the shots and check-up.