There are several ways you can get a static IP to refer to your app. You could deploy a
Managed VM user-managed instance or a
Compute Engine instance at a certain static IP and have it serve HTTP redirects to your appspot domain. You could also look into other technologies. An IP proxy would also work (you could even make it on Compute Engine). A custom domain is also an option, as mentioned, but won't necessarily result in a static IP so much as a static domain name, while the App Engine front-end servers are among a limited address range.