You can root your Google Wifi router, but you'll need a screwdriver
There's an open-source project on GitHub called GaleForce which allows you to customize your Google Wifi router, as spotted by Android Police. Under the hood, Google Wifi is running a version of Chrome OS Linux. GaleForce (Google Wifi's hardware codename is "Gale") lets you run a VPN server, customize your gateway IP address, or do any other system admin sort of thing that Linux makes possible.
The bad news is you'll have to get your hands dirty. To put the router in developer mode, you'll need a screwdriver, a USB-C adapter with Power Delivery, and some precise button presses. But once you do the work, you just insert a USB drive with the GaleForce software on it and you're ready to SSH into your Google Wifi like a real badass.
What's nice about this hack is that GaleForce will keep working even after Google applies automatic updates to the router. It's a best-of-both-worlds situation: your home router becomes a Linux box with endless possibilities, but it still has the mesh networking and app-based interface that Google provides.