I have set up Asterisk for VOIP to VOIP, I want to route calls for allowed users to my GSM/PSTN gateway, has any one been able to do this, and if so is it stable and scalable 
Yes, this is possible. You need to install an h323 driver, as asterisk does not install it by default. Options include ooh323 , oh323 , woomera
CJ … Can’t comment on stability