There are much easier ways to find or check the talkgroups than to do wildcard scanning. 
I use a program called Pro96Com to keep tabs on P25 systems... The same laptop that I have at home providing my feeds also runs this program. Connected to my PSR-500, it keeps watch on the Loudoun County system's control channel. New TG ID's are labelled with the date/time they were first seen, so I'll go through the Talkgroups file from time to time looking for new ones... then I'll add them to my 800 with recording enabled, and after a few days, go through any recordings there may be to see if I can identify anything. This way I don't have to get stuck listening to all kinds of other things I don't want, since I know exactly what TG ID's I want to hear.
There's another (harder to use, IMHO) program called Unitrunker that can do the same, but it also works with Motorola and other systems. I also don't know if the GRE scanners send control channel data for non-P25 systems, since I've never used Unitrunker.
I use a program called Pro96Com to keep tabs on P25 systems... The same laptop that I have at home providing my feeds also runs this program. Connected to my PSR-500, it keeps watch on the Loudoun County system's control channel. New TG ID's are labelled with the date/time they were first seen, so I'll go through the Talkgroups file from time to time looking for new ones... then I'll add them to my 800 with recording enabled, and after a few days, go through any recordings there may be to see if I can identify anything. This way I don't have to get stuck listening to all kinds of other things I don't want, since I know exactly what TG ID's I want to hear.
There's another (harder to use, IMHO) program called Unitrunker that can do the same, but it also works with Motorola and other systems. I also don't know if the GRE scanners send control channel data for non-P25 systems, since I've never used Unitrunker.