Call forward from SIP Sever to GSM Gateway

Hi,
I have VPS (Asterisk SIP Server) and GSM VoIP Gateway. I want to forward my SIP call to the GSM Gateway. Would you please help me in doing the necessary configuration for this. Do I need to configure SIP Truncking?

Thanks in advance.
Regards,
Motaz

This is quite trivial and in majority of situations, the Asterisk sip.conf configuration depends on the gateway.
Try contacting your GSM-VoIP gateway vendor support. In majority of cases they already have the Asterisk and gateway sample configs ready for their customers.