I think the issue with GKs and DE is that neither side really offers much to the other. For a start, neither codex is in a good position at the moment, so you're basically allying 2 bad books (rather than using a good one to strengthen a bad one).
Also, whilst they might go about things in different ways, there's a lot of overlap between their strengths and weaknesses. e.g.:
- Both are good against MCs with poor armour saves (DE have poison, GKs have Force Weapons)
- Both are pretty bad against GCs (GKs are probably a little better, thanks to the Dreadknight and arguably Draigo, but still not great)
- Both are bad against armour
There's maybe a use for allies in terms of DE going MSU to grab objectives and provide long-range fire, whilst GKs go forward. But, it seems like there are far better options for both parties. I mean, you're desperate allies - so not only will you need to keep all DE units more than 6" away from all GK ones, but the DE can't benefit from any GK psychic powers. Nor from any Draigo-teleportation shenanigans. Likewise, you can't use a WWP to get any GK units into position, or any other character-based buff.
It just seems like the GKs would be much better off going with, say, SM allies. They can also do MSU - with small squads of bikes and Scout squads in Land Speeder Storms. But, more than that, they can bring much-needed meltaguns, as well as various long-range weapons. And, you can freely mix and match characters and squads.
Likewise, it seems DE would be much better off allying with Eldar or Corsairs. Both of which can bring far better anti-vehicle than DE, along with various other stuff. Eldar can bring a ton of psychic support (which DE can benefit from), both Eldar and Corsairs can bring high-volume S6 shooting. Eldar even have a better version of the Dreadknight, if that's your thing.
Put simply, GKs offer virtually nothing anything that you can't get via Eldar. But, unlike Eldar, there are far fewer benefits and many more hoops to jump through.