Normally you would reinstall the correct policy to the gateway from the SmartConsole as the_rock said. However if you can't do that for some reason, starting in R81.20 Jumbo 54+ and R82 the gateway itself will cache the last two policies installed prior to the current one. These can be reinstalled right from the gateway itself using policy_rev_tool: sk181437: Access Control Policy Revert Tool (policy_rev_tool)
This is one of the tips in my upcoming CPX Vegas presentation.
Gaia 4.18 (R82) Immersion Tips, Tricks, & Best Practices Video Course
Now Available at https://shadowpeak.com/gaia4-18-immersion-course