If you can find a Peavey Scorpion speaker, that might work out. My Mustang III sounds great through a 2x12 Scorpion cab....The GDEC uses similar modeling..essentially similar voices as the Mustang, I think. Anyway, if one can be found cheap enough, I'd give it a shot. You want an 8ohm speaker as a direct swap. As suggested above, getting it re-coned or replacing the original with the same one would be 1st choice.Any ideas?
http://www.tdpri.com/threads/g-dec-through-real-speakers.174555/
Not sure if you ever found a replacement but I have a GDEC 30 I use mostly for playing Backing tracks and recording. There are a number of older threads on TDPRI about replacing the speakers. Here is a good one.