A switch, unfortunately. This is a problem with GT5p though, not with the G25. They simply haven't programmed it properly to support the clutch.
You can choose to use the gated shifter but skip the clutch. I'll do that sometimes since the clutch is so poorly done (again, this is a problem with GT5p, not with the wheel itself).
Thats a shame, since the whole 'clutch/realism' debate we're having is negated by the fact that with an on/off clutch you don't have the problem of finding the bite point & such, hence artificially speeding up gear changes.