The P25, or APCO25 standard, is just that, a standard. Like EDACS, or any other variety of trunked system types. Within that system there are ID groups that identify the entity on that system. You can have multiple departments on one trunked system. Some of those talk group ID's are encrypted and cannot be monitored on any scanner. The way the GRE product handles it is to displace a letter "E" at the top of the screen and give a busy tone.
There is a wealth of information regarding how these systems work and what it takes to monitor them. It is quiet a lot of reading and playing with the radio, but well worth taking the time to learn and understand. it will make for a much more pleasurable experience with these advanced radios. I basically knew nothing going into the purchase of my GRE, but through spending time reading the Wiki database, and searching through the frequency database to learn what department was on what system, i was able to make a lot of headway. once i ran into some hurdles, i was able to get help in the forums. I really recommend putting that time in, as once you ask questions in the forums it will make peoples answers, suggestion, and help in general easier to understand.
Edit: Tilt, sounds like a good idea. i was able to boot up on one v-scanner and hit menu before it crashed. then the next one that went bunk i would crash immediately. i pulled the card, formatted and did a fresh download for now until i get a bigger card. i am starting to use the data dump which has already filled up the card once.
edit edit: i've really gotten to like this radio better. i did at least get the message so to speak when researching that if you like to tinker the GRE was the way to go. the problem ive got now is that one of the simulcast systems is close by to home, but nearly unreadable unless i use a hi gain yagi. ideally id like to have a dedicated base for that situation, but GRE stuff is up in the air, uniden 996 would be an option, but do i wait to see what comes out in 2014....? ugh....