The Gate of Bones feels like two books of very different quality battling it out.
On one hand you have a pretty engaging, relatively small-scale conflict between the Imperial Guard and a militia of normal, corrupted humans and Chaos Space Marines... but on the other hand, you've got the Adeptus Custodes and the wider story. The best parts of this book were the ones focused on the small players - especially in regards to Dvorgin's struggles to keep his men alive, and Lokk, a Iron Warrior on the verge of true corruption at the hands of the Chaos gods. The more personal nature of the conflict fuels the strongest parts of the books - a tale of vengeance, of desperation, a showcaseof the brutality inflicted upon both sides.
It's when the Adeptus Custodes turn up, filled to the brim with plot armour and being nearly impossible to actually kill, the stakes feel less earned, and the anxiety that Dvorgin and his troops will actually succeed begins to dry up. For the most part, though, I think the prose and character writing of this book is actually very strong - it's just the overall narrative, and the pretense of world-defining factions that poison the well.
As an aside, I also felt The Gate of Bones pull a little bit of a bait and switch, opening with a message from Imperial Nihlus, and I was hoping we'd get a bit more of that nightmare showcased in here, but we pretty rapidly move away from the plot point, likely to be explored in further novels.
The Gate of Bones at times is genuinely fantastic, especially in it's quieter, more introspective moments, and at it's best, frequently outstrips it's predecessor. Unfortunenetly, the elements of the large narrative caused me some level of frustration, degrading this book to 'just another' Warhammer novel. I do really hope some of the characters in this - especially Dvorgin and Lokk - return soon, as they really felt fleshed out compared to the rest of the cast, but as it stands, The Gate of Bones is fine.