Carriers over the past few months have building out and turning on their 5G wireless networks. However, the footprint for each carrier 5G network is still very tiny and limited to a very small number of geographic locations. The largest 5G network, at least in terms of supported cities currently belongs to AT&T. AT&T has a "standards-based mobile 5G+ network" up and running in parts of 21 cities. Verizon has it in over a dozen cities with 30 expected to be lit up with 5G service by the end of 2019. T-Mobile has it turned on in just 6 cities and Sprint has it in parts of 9 cities. Between the 4 major carriers, Sprint appears to be the first one to allow its MVNO partners access to the new network. However, an article in Fierce Wireless says that Sprint is allowing access to the network in just one city at this time, with additional markets scheduled to get it soon.
Sprint's current 5G enabled cities include parts of Atlanta, Chicago, Dallas-Fort Worth, Houston, Kansas City, Los Angeles, New York, Phoenix, and Washington, D.C. The only city where MVNOs have access to it is in Chicago. MVNOs were granted access to it just a few weeks ago.
It is not entirely clear yet how many Sprint MVNOs have been granted access to the network but Google Fi is most likely one of them. Back in February, Sprint sent out a press release stating that they will expand their MVNO partnership with Google to include 5G services. Rumors are also circulating that Google is about to release a Pixel phone that includes support for 5G, so it would certainly make sense for Google to now have access to that network.
According to a check I did earlier, no Sprint MVNO website, including Google Fi, is currently advertising 5G wireless service. However, Allvoi Wireless sent out a press release at the end of August announcing that they would soon make month to month phone plans available over 5G. Altice Mobile should also have access to it either now or in the near future given that they helped build out a network of small cells in partnership with Sprint. I reached out to several Sprint MVNO's to ask for commentary in regards to their plans for 5G but did not hear anything back by article publication time.
Update: Two MVNOs have responded to my inquiry. Gen Mobile has confirmed that it now has access to Sprint's 5G network in certain areas such as Chicago. The rep stated "strategically we will continue to onboard newer and newer devices that support the new bandwidth and will grow in cadence with Sprint." Another MVNO declined to comment on the matter other than to say "not for us."
It's a good move on Sprint's part to grant its MVNO partners access to the new technology. It should help their partners get a headstart on the competition in terms of testing it out and developing business strategies around it. However, in terms of immediate benefit to consumers, it's not going to offer much. The networks aren't big enough and in enough locations yet for consumers to realize a benefit. There also really aren't many 5G enabled phones available for consumers to purchase. Right now the biggest change that 5G can provide to a majority of consumers is to their wallets and not in a good way. 5G enabled phones are expensive, and at least one carrier, Verizon, has hopes of charging customers more for 5G wireless service. Verizon initially launched 5G with the hopes of charging its customers an extra $10/month for the service, a plan that it has at least temporarily scrapped.