Don't think any GPRs exist any more. There are still GFS/GFR products, (Anytime First Sgl/Rtn route EC Only).
NXEC/EC seem to be the only TOC that uses that ticket descriptor so it is possibly for internal purposes so they can easily differentiate when examining ORCATS stats.
Tickets discountable by PRIV are:
All "Anytime" tickets (except flows to Ireland/Non-rail destinations like Jersey C.I.)
Euro Standard Singles/Returns and Euro First Singles/Returns (EOS, EOR, EFS, EFR)
-A First Class TOCNE/PRIV is required to purchase First Class fares
Most regional/national rangers and rovers
Monthly and longer season tickets
Limited amount of attractions such as the Ffestiniog Railway (where through tickets exist)
Tickets that can be bought in association with PRIV are:
Plusbus (Public rate)
Weekend First (Public rate) - available to holders of Standard Class facilities (VT offer a free upgrade with an Anytime Sgl/Rtn)