- This depends on who you want to learn about.
- Gathering Storm (and the supplements you mention) are the starting point.
- The novel series you mention is the new line of books covering the new events.
- This is all relatively new so material is still being released.
So you are correct with the Gathering Storm release and Dark Imperium series as the new material. Gathering Storm is where everything started to kick off and was basically the build up to the "New Imperium" as it is now and the new events moving forward.
As far as the material for you to get your hands on to supplement what you have read from Gathering Storm it depends on what you want to learn. I do not know all of the related content that has been released, but, for example, I have read each of the mini series titles from the Legacy of Russ bundle that cover the events of Curse of the Wulfen and Wrath of Magnus more intimately.
From the Gathering Storm supplements we know that the Space Wolves ran around the galaxy collecting the Wulfen and then ran home to protect the Fenrisian system after the Thousand Sons invaded. The mini series goes into more detail of what planets were attacked, how and by whom (the Death Guard were there too), the rituals performed on them, how the Space Wolves fought back, etc. It appears that on Black Library the line was combined with a couple others and is available here or each of the lines can be purchased separately...
As Nino has mentioned there are other titles that cover events that occurred in other parts and factions of the galaxy. My recommendation is to travel to the Black Library website or maybe to Lexicanum to search for factions or events you are interested in and scroll down to see their sources. Again, for example, the new Role Playing Game Wrath & Glory is set during the events of the Dark Imperium and there may be details of interest to you within that release...