To gem for summon a player has two option.
One, to pay 300 gems for 1 Rare 3-5* summon
Two, to pay 2700 gems for 10 Rare 3-5* summon (Buy 9 get 1 free!)
While every day the Featured unit changes, it has a possibility though it may be low to repeat.
The featured unit gives the user an increased chance to obtain the unit, this increased chance to obtain can come in the form of any Rarity 3-5*, and of any Hero Type (Lord, Breaker, Warden, Anima, Oracle.)
A player may also acquire a "Free Rare Summon" card from quests, or chests which may be used to substitute a free summon in place of spending gems.