Use command unlockeverything to... you know, unlock everything.

Use unlockeverything disablesteamachievements to prevent steam achievements from being affected (this only applies for the current session, so if you close the game and re-open the achievements may be affected)

Installation
Get MTG:

• Download Mod the Gungeon at: http://modthegungeon.eu/#download
  
• Make sure Enter the Gungeon is closed
  
• Open ETGMod.Installer.exe and press "Step 3: Install ETGMod"
  
• Close dat thang
  

Installing mods:

• Go to your Enter the Gungeon Steam directory, usually located at:
  "C:\Program Files (x86)\Steam\steamapps\common\Enter the Gungeon\"
  
• If there is not one already, create a folder called "Mods"
  
• Drag FilthyCheaterMod.zip into the Mods folder
  
• If there is a "mods.txt" file in the Mods folder, delete it
  
• The directory should look like this:  "Enter The Gungeon/Mods/FilthyCheaterMod.zip
  

Then launch the game. If Mod the Gungeon installed correctly, you can press F1 to see the list of active mods. 

Open the console with ~ or / to enter commands.
Type help for a list of commands.