GFL is probably one of the best gatchas out there. It isn't overbearing with asking for money. In fact, the only things you really pay for are infrastructure (ie active squads, build slots, storage space, etc (these are all 1 time purchases)) and cosmetics. All story and events are free without even the option to pay for help and they give you plenty of time to be able to complete them. Even unit and equipment management is all gameplay driven.
You can buy resources with real money, but it generally isn't worth doing it, especially since it isn't hard to simply farm the resources on your own with logistics.
As far as this collab goes, they have been talking about it in the EN community for a while. In fact, it's the reason why I went out and bought the Gunslinger Girl Blurays to watch.
Unfortunately, I didn't really like it all that much. It seemed like it had no plot and was just a slice-of-life from the perspective a bleak world where they brainwash little girls and use them as assassins. But, even still, I'm looking forward to see what comes of this.
Edit: I just realized after looking at my post: my avatar is from GFL :3