GPIO Write value?

The Fastrack Suprema has two pins: GPIO21 and GPIO25.
The AT Commands Interface Guide says that it can write (or read) up to 10 GPIO ports (going from 0 to 9).
Should I use the command AT+WIOW using the ports 21 and 25? The guide says that I can only use from 0 to 9.

What I want to do is write a ’ reset state - zero’ to GPIO25 when a given SMS is received, and ‘set - 1’ the same port when another SMS is received.
Am I doing this write!? Does anyone have any suggestions!?
Thanks in advance.