Gems arrive from drops while you clear the platform, from selling spare items to the Gem Dealer, and from codes. They are the only currency the ability wheel takes, which is what makes a roll a real decision rather than a purchase.
The roll odds are not published. The wheel shows you the rarity tier of what you got, but the developer has released no drop table and no pity counter. We are not going to print percentages we cannot point at a source for — that is the one thing this page got wrong before, and it is worth more to say so plainly than to fill the gap with a number.
Cooldowns and durations are the same story. How long an ability lasts and how long you wait for the next cast are readable in game on the ability itself, and you should check them there; this page describes what each ability does rather than promising timings it cannot stand behind.