It's a bizarre and alien state of affairs.
One of our better known comical excursions, "I Hope You Like Text," is (among other things) about how Gabe has no idea how vast the Games Workshop legacy is. People are filing off those serials all the time. Of course, because it's a creative endeavor, their own settings have some Dune and Foundation in them, but why shouldn't they? It'd be weird if they didn't; that shit kicks ass.
The Wargame he got the deepest into was Warmachine, which has some cool fluff in it, but 40k fluff is on a completely other level. I don't think it requires that you want to paint tiny mans at all, in any way. Not every book is good as the best ones, but the best ones I would put against just about any modern sci-fi - purely as books - without getting into the force multiplication of the many, many material or digital games they're connected with. Indie wargames are in a very good place right now, and I play those also, but there is a firehose of releases for multiple product lines and if you are looking for a community, it's a numbers game - this is where they are.
Personally, I hope he goes with Ultramarines because I want desperately to kill them - they're sort of the face of the setting from a visual perspective, with a reputation for being tight-assed schoolboys… Except the book Know No Fear really turned me around on them. Prospero Burns did the same for Space Wolves, which are essentially Vikings, and I was highly resistant to Space Vikings. Both of those books are by Dan Abnett, which is no surprise; he's also the guy Fatshark tapped to do the writing on Darktide, and there's no time like the present to find out why.