It's worth noting during the great crusade Gulliman was arguably the most sucessful of the primarchs, others may have conquered more (although that seems to be debated from source to source) but Gulliman laid the best foundations (well except literally, then that was Dorn

) I honestly doubt we'll see much chance of Gulliman shining, the few
HH books that have focused heavily on politics (unremembered empire for example) have often been dismissed as "dull" which is sad as honestly I find that stuff more intreasting then the bolter porn that most
40k novels are.
I just suspect that we won't see much of the post Heresy materiasl, although I'm sure you could write a few good novels about the personal conflcit between Gulliman and Dorn