Yes, the JP Enterprises .156 Oversize Anti-Walk Pins will work with the GMR-13.
Yes. The JP Enterprises .156 Oversize Anti-Walk Pins come as a set of two. No modification to the lower receiver is needed, but be aware that the trigger and hammer may have to be reamed for a perfect fit with these parts, and a detail stripping of the lower receiver will be required in most cases.
I would recommend reaching out directly to JP Enterprises for a replacement. You can find their contact information linked below:
https://www.jprifles.com/2.1.php
Yes, these should work for that application.
Rise Armament recommends using KNS trigger pins which are .154
These JP .156 pins can still be used, but some fitting may be required