I have deployed an asterisk PBX. I want to be able to go to the PSTN network using a VoIP Gateway. How do I do it? Please give a detailed answer.

Hi! if your gateway supports the SIP protocol, you could setup a trunk between it and your PBX. Right after that you should modify your dialplan to route calls with your local number format (_05ZXXXXXXX for exemple is the local dial pattern in morrocco) through the trunk you setted up, plug the PSTN line in one of your FXO port and that should do the job.

Hope it is clear enough! 

Good luck