When in Game Guardian click the hamburger menu, scroll down to the execute script and navigate to the location you saved the script to.
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added 0 minutes later
I am unable to get the unlock all cars script to work. It completes and says it's applied, but nothing changed. The other scripts work, but causes the game to slow to about half speed, so do what you need to with the script running and then restart the game. Could just be my phone. I've seen this behavior before in other games while using gameguardian.