Google Play - https://play.google.com/store/apps/details?id=com.cjenm.sknights
iOS - https://itunes.apple.com/us/app/sebeunnaicheu-for-kakao/id741949324?mt=8
Facebook fan group - https://www.facebook.com/groups/7KnightsGame/
You can easily get donate gems just by little grinding and there not many things that really worth it. 250 gems - 10 random 3-6 star cards, most time it going be only 3 star card. Costumes cost 150-300 gems but most of them look bad for me. There also passive player skills that cost gems but they give not much and I'm not sure how they work atm. You also can buy keys for gems to play in dungeons but they cost almost free. For 150 gems you can run again in the elemental dungeon, be sure that you do it on the right day.