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Are they competitive? Or are they too slow on the table to compete?

Is their fluff cool?

They are lower mid tier faction mainly due low unit count and number of grav in game now cullexus does ruin their day too. There's still couple of fun lists when allied with inquisition or space marines for drop pod purifiers.
Gk itself lacks any good long range support due purgation squad taking precious NDK heavy spots.
Fluff wise it's a matter of perspective. The idea of marines focused on dealing with doomsday level of demonic menace works for me.

Furthermore, nearly all fluff regarding them revolves around allies being involved. Fact is I still feel like they work best as a supplement to guard and sisters as it just fits the fluff...

Well my main army is imperial guard.

I use mine with inquisitors and tempestus scions. Gives an "elite" feel, even more so than just knights. As someone said, by themselves they are lower mid tier. Allied, they bump up to top tier. Lower top, but top

Well they do benefit from allies indeed lots of fluffy list's will involve
Inquisition -in game they can boost your av and range support. Fluff wise it's common ordo malleus inquisitor will lead a detachment of grey knights.

Red hunters space marines - gives you drop pods and peeps to hold objectives support from range.
In fluff it's a chapter with very close ties to inquisition so they did and will serv together with grey knights when needed.

Guards - lots of bodies to complement numbers of gk plus av and range. Valky purifiers are sweet too.
In fluff every planet has a pdf so there's allways a chance gks will tag along if needed. However it ends badly for guards. Like allways.

Knights - all what you lack as gk can be made up by one of thise .Fluff wise there might be a freeblade that tags together with gk or maybe even a freeblade honorbound to inquisition or ordo malleus.

Yea OP you would be a great fit for moving into the direction of adding some Grey knights... which also gives you access to the storm raven, and honestly if you read the fluff about them they have a very fun back story. Also, you could look at another route and instead of grey knights you could add some marines as Death Watch Marines (Awesome fluff, and just a couple squads still makes tons of sense fighting beside your guard.)

I play pure Grey Knights at 1500 points and lower, no vehicles, just TDA, a few PA, and 2 TDA monstrous creatures. They are fun, fast, built more to win Maelstrom than to tabling their opponent. Few armies can match GK in the Psychic phase. Been finding them a blast to play since their 3rd Ed codex.

SJ