Gems only buy summons, so every gem question is really a question about how much a banner pull is worth to you. Here is where they come from, ranked by efficiency.
The sources, ranked
1. Codes (one-time, biggest burst)
The live codes together are worth roughly 12,000 gems plus a pile of crystals and tokens. If you have not redeemed all ten, that is the single biggest gem injection available to you and it takes four minutes.
Level gates matter: GALAXYEVENT, GALAXYGEMS, 2KCCU and 3KCCU need level 25, and INVITED needs level 100.
2. Nightmare story first clears
The best repeatable-until-exhausted source. Each act’s first Nightmare clear pays a large gem bundle, and there are enough acts to fund a full banner rotation.
3. Challenges
Modest gems per win, but they also pay the DNA and tokens you need anyway, which makes them the most efficient “gems as a side effect” mode.
4. Season Pass and level milestones
Fixed payouts for playing normally. Check the profile screen’s milestone list after every session — it is the reward track players forget.
5. Endless
Endless pays on run completion, not per wave, so a run that dies at wave 12 is poor value. It is a gem source for players who can already go deep.
What to spend gems on
Banner rotations, not the standard pool
Rotating banners are where Secret units appear (Roger is the confirmed banner Secret). Spending everything on the standard pool in week one is the classic mistake.
Pity awareness
Secret pity was added October 24, so long dry streaks now build toward a guaranteed Secret. If you are close to pity on a rotation you want, finish it rather than splitting pulls across two banners.
Event capsules are not gems
The MUI Goku event uses Rainbow Stars, not gems. Do not convert gems into stars — stars come from playing the event and from codes.
What not to do
Do not buy coins for gems
There is no gem-to-coin conversion worth using. Coins come from runs.
Do not reroll stats with gems
Rerolls use Stat Tokens. If a guide tells you to buy tokens with gems, it is describing a different game.
Do not chase Secrets with a starter team
A Secret on a team with no farm and no control still loses to Nightmare waves. Build the frame first, then the carry.
A realistic first-week budget
Days 1-2
Redeem everything, clear story normal, spend the first 10-pull, keep 0 gems.
Days 3-5
Nightmare first clears on the acts you can finish, challenges on rotation. Bank the gems.
Days 6-7
Spend the bank on the current rotation if it contains a carry you need; otherwise keep banking and grab the level-25 code batch.
For the full currency picture, see progression and currencies.