Head over to Settings > General > APN:
netgear-apn
Click Add to create a new cellular profile
netgear-add-gfi
For the network Name, enter Google Fi. For the APN enter h2g2.
Click Save to add the profile
Then make sure to select the correct radio button to activate the new profile:
netgear-activate-profile
At this point, I would recommend deleting the old/default T-Mobile profile. I would also suggest rebooting the device to force it to switch to Google Fi.