Once you have setup Domain Forwarding Service for your domain name, you need to modify your Name Servers to the ones specified by Gibraltar .GI Registry. Generally, the Domain Forwarding Service should start working within a span of 24-48 hours post modification of Name Servers.
Additional Information
Do not be confused when you encounter errors such as No NS A records at nameservers while using some DNS Tools like DNSReport.com. This does not indicate an error and will not affect the Domain Forwarding of your domain name.