Call forward from SIP Sever to GSM Gateway

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.

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.