If by USG you mean the USSS (Secret Service) sight, then yes, it has tritium in it. There is no light collector in the USSS sight. The daytime reticule is a black "T" with a large black circle around the outside and a small circle with a 1 MOA dot in the center. In low light the "T" glows a dim red so the weapon can still be aimed effectively after the sun goes down.
I've always been confused with those terms too. I heard the USG means United States Government. Which means it was altered from its original specifications to meet US standards. But I've also heard that the USG is the original one with the white ring sight, and that's where the USSS with the black reticule comes into play. The Secret Service wanted a different sight that was a little more visible so they requested a change to the sight and it became the USSS sight. I guess I'm confused by the two terms, are they the same gun just different names? Or is it like the Five seveN where the USG is the standard and the IOM is the older one?