This app was mentioned in 2 comments, with an average of 7.00 upvotes
You can try at Play Store, maybe if you use a VPN you can manage to download it, or if you know a good site to transform playstore link into apk (saw one of these once, if I had it yet I could send you) or you can download from oficial website for PC, you will need to download AndApp and then the game (I can be mistaken, maybe you can play directly from browser just like Ganblue).
I was trying to take the APK Pure apk for you but didn't find it, remember to take mine there once, if I manage to find it I'll just make an update here.
The promos mentioned here are the ones in the Extras tab on the main menu. By playing the Rage of Bahamut and The Idolm@ster: Cinderella Girls mobile games with the same account as your Granblue Fantasy one, you can get some full elixirs and premium tickets. If you haven't already, and don't mind wasting a couple of hours doing some mindless clicking for more of those hot GBF items, you may want to do so before the promo ends.
This is a guide for Rage of Bahamut and this is a guide for Cinderella Girls (Although the Cinderella girls one is based on the collaboration events rather than just this promo, so there's extra stuff in it that you don't need to do to get all the promo rewards.)
EDIT: This is the link to the mobile browser version of Rage of Bahamut, and this is the link to the itunes store location of the Cinderella Girls game. I personally found the chrome apps a bit easier to deal with when I messed with these once upon a time, however, so I'll also link them. RoB Chrome. IMCG Chrome.