I don't know for a fact whether or not there are commissioned officers in CGIS, but there are definitely enlisted and CWO's. You need to be an E-6 in order to apply for it, and CGIS does a lot of their own recruiting. They look for prior law enforcement experience. It's not NCIS though. They have the ability to investigate CG personnel and things that happen on CG bases, but they don't have the broad jurisdiction like NCIS has. NCIS is very comparable to the FBI. I'm sure that CGIS has it's good and bad features. I personally wouldn't want to do it. The idea of busting Coasties never really appealed to me. It is very selective to apply for it. I believe that the reserve rating of IV is associated with CGIS.
"To me, clowns aren't funny. In fact, they're kinda scary. I've wondered where this started, and I think it goes back to the time I went to the circus and a clown killed my dad." - Deep Thought by Jack Handey