Easy. Restore your test machine to starting state and try again with your GPO but with only a smaller number of well researched changes at a time.
Oh, you are experimenting on your production network? First lesson, ALWAYS experiment and test off the production network. VMs are your friend.
Almost as easy. Restore from your system state backup.
Oh, you didn't make one. Next lesson, always make a backup.
So now you need to reverse a change. Implement your rollback plan.
Oh, you don't have one. Third lesson, plan, develop, and test your rollback plan as you test your changes.
Now you're in recovery mode and hoping nobody notices. Doesn't feel good eh?
You need to keep the group policies you added as a reference.
Disable this group policy.
Log onto another machine and build a group policy which explicitly
allows everything you denied.
If there are any policies which do not have an explicit allow, the
new group policy should at least set it 'undefined'.
Apply this group policy.
Any policies which did not offer an explicit 'allow' will need to be
This sort of experimentation belongs on a test machine on a network that is not connected to your main network. As you're developing and testing, plan and test your rollback plan as well. Once you've tested the GPO you want to apply, don't forget to back up the system state of the production server before applying the new policy. And try to roll out in stages, since such policies often have unintended consequences, and each server is different. System state backups will save your bacon, as will testing and staged rollouts.
As you have learned GPOs are very powerful, and sometimes tricky to reverse.