NO expression pedal will work right with the G5. You need a VOLUME pedal for G5 expression. If you look on the back of the G5 near the exp jack it gives two model numbers for the two korg volume pedals that they recommend. I believe the specs for them are 250k. If you use anything with less resistance you won't get the full filter sweep.
That said, the two volume pedals that they recommend are impossible to find. I picked up a DOD VCC1 volume pedal recently and it works great. Only downside is that it has a 450k pot so there is a little bit of a dead zone near the heel where it's giving more resistance than needed for the max sweep. Going to see what i can do to fine-tune things but it gets full functionality so i'm happy.