EDIT
Wait, t-mobile is GSM and sprint is CDMA; google would use two different cell antenna technologies? Guess we'll need more dual-antennae phones.
It is already being done in Verizon and Sprint phones. Verizon's newer phones are world capable out of the box. Sprint now has an LTE network. LTE is a GSM technology.
Google asked the FCC for spectrum in the 2.5Ghz and 2.6Ghz ranges back in early 2013. I don't recall if they ever got it. However, Google would have the same issue that T-Mobile and Sprint currently face: No low frequency spectrum (700Mhz, 800Mhz and 900Mhz). Both AT&T and Verizon own spectrum in these bands allowing them to have better signal penetration in rural areas covered by hills, ridges and foliage.
At best, if Google did get the aforementioned spectrum, it would only be ideal in areas where subscribers have LOS or nLOS to the tower.
What Google is doing now with T-Mobile and Sprint is not much different (or is it?) than using a service like Straight Talk of NET10 Wireless.