All fuel guns leak a little.
I think the ATS are made by some company in Italy and I have never heard of a rebuild kit for them. All fuel guns are the same in principle design and there are only 3 o-rings in them anyways that I recall. The one on the plunger where the fuel comes out, one at the cap where you fill it, and then there is the big skinny one on the body where the two halves separate. You should be able to find equivalent o-rings at most well stocked auto parts stores.
If the big skinny o-ring doesn't seal that well, you may want to clean everything really good and apply a thin layer of O2 sensor safe silicone where the halves join.
Remember that fuel guns are open by design and cannot be layed down when full or they will leak no matter what. You must keep the outlet of the gun pointed down all the time.