List GWGs in Interface to send update from

Back-port of this fix done in master https://github.com/pfsense/pfsense/commit/31300a95f71b14dcb98c139388205223a36e8c8b and https://github.com/pfsense/pfsense/commit/8f56dd279432c4fd5a027310622e2650822e4651
Unfortunately this never got back-merged to 2.1 branch. A user on the forum noticed the issue again on 2.1.1 - https://forum.pfsense.org/index.php?topic=74922.0
I am submitting this so that when the next security issue happens, and there is 2.1.n+1 then this little fix will also happen automagically. I wonder what other fixes are in master that missed being back-ported to 2.1 branch.