Thanks Tuomas!
I have already sent mail to GMP. They said their batteries are not fully compatible with the IDX charger and it was best if I used the GMP charger.
They also said it would work, but not optimally. GMP: "Using incorrect charge will likely cut the
battery life span in times."
I think I'll try the combination IDX charger and GMP batteries to begin with. And replace the charger if it's not working properly.