Gatekeeper allows administrators to take a WordPress site offline while leaving it accessible to authorized users for maintenance, testing, etc.
I needed a "coming soon" page for a site I was working on but also needed to be able to use WordPress myself in order to test themes, designs, etc. The existing landing page themes are nice, unless you also need to be able to test a theme without letting the world see. At first, I added some PHP code into the WordPress index.php file to redirect all but my IP address to the coming soon page, but WP upgrades opened my site to the world. Editing the htaccess file is an option, but it's extra work and only works until my IP address changes. So...I created Gatekeeper.
No. Gatekeeper protects all WordPress pages (except admin and login pages) from everyone but those whitelisted, so redirecting to a WordPress page would create an undesirable redirect loop.
The offline page can be any web-friendly page: HTML, PHP, text, etc. It's not required, but recommended, to keep this file in a separate directory to keep it from being confused with WordPress files.