OK, by using it for GPS, you are using gigabytes of data, I think.
With Googlefi, the basic charge each month is $15 for the 1st person on the account and $10 for each other person on the account. That basic charge includes unlimited phone and text.
The additional charge for data is $10 per gigabyte. You are only charged for the gigabytes used. IOW, if you use 1/4 of gigabyte, you are charged $2.50. If you use 1.75 gigabytes, you are charged $17.50, for the data in addition to the basic charge.
My wife and I used Virgin Mobile for years. Our account cost us $75 per month for the two us us, plus government taxes and fees. Since moving to Googlefi, our monthly bill, including government taxes and fees, has generally been below $50. Only 2 or 3 times has it exceeded $50.
You can purchase a Googlefi phone directly from Googlefi. On Black Friday, they had one model smart phone available for $149. There are also twelve (12) models of phones available that you can purchase from other sources and use them on the Googlefi network. They will give you such a list, if you ask. However, be careful because many other models of smart phone can not be used on their network. For some of those 12 models, only some of them can be used. As an example, some Pixel 2 and Pixel 3 models can be used and others can not be used. For other models, only those sold in the U.S. can be used. If you purchased one in another country and imported it into the U.S. you may not be able to use it.
What I like best about Googlefi is that it can be used on both of the networks that exist in the U. S. and also on WiFi. If my wife receives a call while traveling in our car, it will switch back and for the between the best signal source while I am driving.