Here are the links for the 3 collab games, which reward 65oo gems after completing all their tutorials. All links are NSFW. If you're outside Japan, it seems you have to play the .co.jp (18+) versions. You have to play the games with the same DMM account you use to play FKG.

https://pc-play.games.dmm.co.jp/play/monmusutdx/
https://pc-play.games.dmm.co.jp/play/MistTrainGirlsX/
https://pc-play.games.dmm.co.jp/play...estarknightsx/