With editor you prefer open /etc/asterisk/sip.conf for editing. Add next on back of [general] section:

register => sip_1:redhat@192.168.0.5/501
[sip_1]
type=peer
host=192.168.0.5
fromuser=501 in dial plan, lines which start with 501 are coming from GSM gw
fromdomain=192.168.0.5 IP address for GSM gw
secret=redhat password for auth
context=default
dtmfmode=rfc2833
disallow=all
allow=all
canreinvite=no
nat=no
insecure=invite;very
qualify=3600

On GSM side you should got something like next picture. We have One Stream gsm gateway