Gems are the "premium" currency in Gumballs and Dungeons, they can be earned in a variety of ways aside being bought from the Shop
Gems obtainable on regular basis (unlimited sources):
Login bonus. 3 gems twice a week, it is recommended to complete ASAP the fate link between Checkers and B&W Queen to upgrade this amount to 4 gems.
Circus quests. Complete 10 circus quests for 10 gems reward (once every 5 days).
Wilderness exploration in Alchemy tab. Keep them active! Random amount of gems (3-6 gems daily as average).
Fate links. Any fate link, even random, has a chance to give you 1 gem.
Sky titles “Junkman” and “Gravedigger” give a chance of gems while searching junk/corpses in sky exploration, these titles are very easy to obtain, just buy the related runes in the Sky Market.
Divine Dragon's wish: I want gems! (6 gems) Maximum 1 gem wish per day.
Alliance Missions. Mission "Host Collection Exhibition" gives a gem when completed. Requires 3 out of Jeweler, Merchant, Divine Dragon, Archaeologist, King
Alliance Credit Gift Packages. 1st pack: 2000 pts 1-2 gems, 2nd pack: 5200 pts 1-5 gems, 3rd pack: 15600 pts 5-15 gems. Gems from packs are random and not related to your personal contribution. Points are the sum of the daily spendings (gems and gold) of all the alliance members. It is recommended to coordinate with your alliance mates in order to take maximum profit each day and avoid to waste spending points.
Purchase Workshop Signboard (Best money to gems deal in game). Collect 50 gems once +10 gems per day for 30 days (support the game!), upgrade it to Silver signboard (cost 300 gems) which gives 11 gems per day and +100 days validity, then save upgrade it to Golden signboard (cost 600 gems) to get 12 gems per day +200 days validity. If you have Alchemist gumball each day there is a chance to get additional 5 gems.
Buy them directly (recharge). Check your time-limited daily packs for best efficiency!
Shopping Rebate Gift Packages. If you or one of your alliance mates buy gems (recharge) there is a chance to obtain additional gems in these packages.
Complete story mazes (pre-endless mode). 1 gem each.
Maze specific gems. Ancient arena (kill audience), Lost temple (statue of Aioria), Forest of whispers (frost mushroom), Desert oasis (genie wish with holy lamp oil), Skeleton island (ghost crystals), Hell frontier (using blue crystals on platforms), Erathia (genie wish in satellite (not related to Desert oasis’ wishes), Decour’s safe), Spacecraft ruins (see related doc).
Event mazes. 1 gem per difficulty level once per event plus event related easter eggs (see events doc).
Underground treasure/Foreign businessman. Chance for gems under the slates and in frog’s shop respectively.
Gumball specific gems. Some gumballs give a limited amount of gems when used: for example Adventurer, Pinocchio, Pirate, Slave, Jeweler and Rabbit. (see skill compendium doc for a full updated list and how to get them).
Gumball trials. Some gumballs give gems after completing their own trial.
Gate of creation gems. Unlock Peter to get a huge amount of gem inside the gate of creation! Time Wizard required.
Sky instances. (see related doc, a lot of gems here!).
World tree rewards (fruits).
Mysterious statue rewards (mysterious statues).
Faction Trial Towers rewards.
Spending return packages.
Star rank rewards.
Snowy mountain rewards.
Repair Eden buildings. Some of them have gems as reward.