Two economies, one screen
Gold is the currency that survives the run. Gems decide the run itself. Keep the two separate in your head or you will misread every choice.
You earn gold from kills and from surviving to certain points. The total you bank funds the menu outside the run.
The mid-run merchant
Some stages open a shop during the run. It sells weapons, passives and healing for gold. Prices climb the longer you stay alive.
That climbing price is the trap. A late shop item costs more than it did ten minutes earlier, and your build may not need it.
Save for the menu
The menu spends gold on permanent unlocks. Those unlocks change every future run, not this one alone.
Spend there before you spend in a shop. The money farming page shows how to build a gold surplus fast.
Next: How to pick your stage.