To unlock it simply click on gnome's right eye 5 times on the title screen.
Here example image on the right.
If done correctly you will see text saying Cheats Mode at the top part of the screen. You can deactivate it the same way.

When playing maps you can activate cheats by any of the following commands below:

• K = Adds 100 Gold
  
• B = Adds 100 Banish
  
• L = Adds 1 HP
  
• P = Skip day
  
• O = Reset day (a bit different from base reset day function, for example it will not remove purchased items)
  
• J = Reroll shop
  
• M = Unbanish all relics
  
• C = Banish all Common items
  
• U = Banish all Uncommon items
  
• R = Banish all Rare items

Information is from Gnomes wikipedia:
https://gnomes.wiki.gg/wiki/Cheats_Mode