Howdy,
Your thread is looking lonely, so I'll take a stab at it.
"GC", is an acronym for "Group Call" (to an entire talk group) as opposed to an individual call (one radio to another). I don't like acronyms either. I guess they were wanting to keep the file size of the program low...
My guess is the traffic is Time Division Multiple Access (i.e. P25 Phase 2) or it is encrypted. Basically, it's something that DSD is not supported to decode.
More information would be useful to help you out. Are you using DSD or DSD+? Which version? What is your SDR/software setup? What system are you monitoring? What happens when this even occurs?