I can't really provide much support for Google Fiber specifics, but based on a quick search, it looks like that is made by Technicolor (https://fcc.report/FCC-ID/RSE-FGA5330/4705137.pdf ) and is a full router and WiFi access point. This doesn't change how you can connect multiple Google WiFi units to each other (they still can't connect in parallel to the same outer network). But, it does mean you'll be living with double NAT, and if your fiber service is faster than 1Gbps, putting everything behind the primary Google WiFi unit will limit your network to 1Gbps, since those units only have 1Gbps Ethernet ports.
So, using this combination of devices may not be ideal. If you want your wired network to be able to use the 2Gbps aggregate speed without bottlenecking through the primary Google WiFi unit, there are a variety of options, but frankly none of them are perfect. The simplest would be to just use the wireless mesh interconnect between the Google WiFi units, connecting the first one to your wired network somewhere close to the center of your home and then placing the secondaries one or two rooms away from there. This isn't ideal, though, because it will still be a separate logical network behind a firewall. So, your wireless devices won't be able to "see" wired devices, and wired devices won't be able to connect to wireless devices at all without some extra work. If you can run more Ethernet cables, you can improve the performance of this approach by wiring those secondaries into the primary (1st Google WiFi), but you still can't connect them all in parallel to the same outer network.
...that is, unless you decide to use them in bridge mode. This is fairly painful to set up (each Google Home "instance" can only have one network, so you'd need to create multiple Google Home instances, and then add each Google WiFi unit in bridge mode to each one separately). They can have the same network name and password as each other. But – and this is the important issue – this approach will break the hand-off support between each Google WiFi unit. So, your WiFi clients will be more likely to get "stuck" talking to one access point they can still see, even when they've been moved to a spot closer to one of the other access points. It also means you lose a number of other Google WiFi features. So, we don't generally recommend this approach – you might as well invest in something like an Ubiquiti system at that point, since it can be deployed like this without losing the hand-off features.
I hope this makes sense – home networking is complicated, and moving up to faster than 1Gbps speeds makes things even more complex right now, since a lot of equipment isn't ready for that yet.
