Gatekeeper redirects OpenACS site requests to a remote URL. Each instance of gatekeeper works for a single URL. To install:
Go to http://yourserver.test/admin/applications
Click "Add Application"
Choose "GateKeeper" and optionally fill out the other fields, and click OK. If gatekeeper doesn't appear on the list, go to http://yourserver.test/acs-admin/install to install it and then resume this process
You should now see an "Applications" page with a list of applications including your new gatekeeper. Click Parameters for your new package.
Enter the remote URL as GuardUrl.