The Gem Letter is one of the gem packs available for purchase. In the Gem Shop, the Gem Letter is the pack with a pink background, and shows a mailbox stuffed with gems.
The Gem Letter is different from other packages in that, rather than giving a one-time lump sum of gems, it instead delivers 70 gems to you every day for 30 consecutive days, which are collected from the Daily Quests. Please note that the gems are not automatically added to your balance, and must be collected daily. Any gems that are not collected will be lost!