I'm not familiar with the GRE's so I can only guess, but I think I remember them being similar to the RS scanners, which allow a mix of Motorola/Trunk and normal Analog channels, so you should be ok.
I usually try to scan until I know they are there, then I either put the scanner on hold on the primary, or break the squelch, so it stays on the channel..I seem to hear most of what they say when I do that, otherwise it can be hit or miss, theyre chatter is usually quiet, but depending on the weather like last night, they can be chatting a bit, plus some just chat more..
I kinda cheat tho, I have a scanner just for air/milair
fyi, I heard Ethel70 checking in with Moncton a while back while I was writing this, I would guess they are going to do something soon if you have your ears on.