Google Fi customers can already use T-Mobile's 5G network1
Andrew Martonik is using a Samsung Galaxy S20 Ultra on Google Fi and he noticed that his phone switched to T-Mobile's 5G network in New York City.
The problem is the speeds achieved by the phone (135Mbps download / 42Mbps upload) on T-Mobile's network are far from being 5G speeds. Nevertheless, it looks like the cellular indicator does show that the phone is connected to the “5G/LTE/3G/2G” network.
Furthermore, Andrew mentions that his phone's diagnostic screen confirmed it was on Band N261, which is mmWave confirmed by the carrier to be available in small areas of select cities.
Of course, you won't be able to take advantage of blazing-fast speeds on your “Designed for Fi” smartphone unless it supports T-Mobile's 5G networks.