Open the Epic game launcher. On the left bottom side, you should see settings. Open it. Then scroll down until you see a list of games installed. Open the one for Galactic Civilizations 3. Tick the box beside additional command line arguments. It then should then open a text box. Add '-cheat' (without quotes) into the text box. That should be it.
To use in game, start a new game (this does not work anywhere except the main map), then press the '~' key just above the tab key. If you type 'help', it should list the commands available to you. Or you could go and read an online guide.
Edit: Oh, wait, you fixed it yourself.