Gulu Mine
From Capybara Go! Wiki and Guides
Gule Mine is a new Challenge mode since v1.7. It's a source of Equipment Gems and alternative to Abyssal Realm (which was originally announced to be more seasonal). The game play is closer to Hard Mode, albeit less packed. But enjoining a team mate (coop mode) is obligatory here.
Participating requires a ticket. You get one free per day. But might be able to get invited per notification (rewards decrease for free joins).
Besides direct gem drops, there's also Treasure Keys now, for the new tab besides the Equipment Shop.