I'd like to get my girlfriend into 40K...

It's generally not cool to go right ahead and buy someone something from your own hobby, because if they're not into it they have a balancing act of not offending you but also expressing they don't like it.

So start with telling fun fluff stories if any are related to her interests. Build up from there.

tell her you'd like to play with her, tell her to give it a go, flick through the various armies and explain how they work. proceed from here.

My GF really likes the inquisition and I play eldar so for her starter she asked to go 50/50 with me on the Death Masque game, figure it'll be good because it has a 'story mode' as well as a little rule book and some really cool miniatures.

Chaos. Try Slaanesh, might improve your fun in other ways...

I got my ex into 40k via nids. She loved those hungry fucking bugs

What's up with that? Does the idea of hivefleets birthing forth vast armies that later return to the womb trigger the maternal instinct?

Probably because they are pretty much the only asexual army. Well, maybe tzeentch too.

I think it's more about them being "cute" critters rather than soldiers. It's like collecting cats or something.

but user what's cooler than the human spirit made to combat?

tiny critters apparently

For the most part, women relate to the Tyranids because women are ravenous monsters who only exist to consume. After they have drained you of everything, they will leave and find another victim and repeat

Or they're just cute. My ex found rippers adorable.

Tell me more about this ex of yours. She remind you of a hive tyrant at all?

They sympathise with the inhuman all-devourign monsters that birth out hordes of ravenous spawn only to consume them along with everything else. Then again, doesn't everybody?

Really, I think it's because girls are less into military stuff than guys, so tanks and mechs don't appeal to them as much. Nids are probably the least like a traditional army, with just lots of critters instead of normal tanks and artillery.
Plus, they also offer a lot of painting opportunities. The female 40k players I know are all more into the painting and modelling side of the hobby, and the ones that don't have Nids have other armies that can be painted in vibrant and eyecatching colourschmes, like Eldar and Daemons.

Also, Nids are surprisingly cute.

who the fuck doesn't do their best to make the most autistically detailed paintjobs and custom builds for their armies? when you're first shelling out money for everything why wouldn't you do it properly?

You'd be surprised. I know people who prime on the sprue and never move past that.

that's me! I'd play with bits of paper if anybody would accept me.

She must've been a termagant.

For some reason, a lot of 'nids are named after unflattering terms for women. Termagant, crone, harridan and harpy come to mind, and there's also hierodule which is ancient Greek term for temple prostitute.
That's almost as many as ones named after Latin words that don't make any sense (zoanthrope essentially means were-animal, haruspex is, IIRC, somebody who predicts the future from intestines, tyranofex means something like tyrant-makers, hierophant is Greek word for a priest...)