One way but not using the Background Tasks extension: HOW TO: use LocationService extension Version 1 and Version 3 with a GoogleSpreadsheet to record real time locations while the app is in the background shows one way to read the gps coordinate values and a way to save those coordinates.
You might wish also to read Background Tasks extension [3.8 A] 🥳 - #38 by SteveJG with respect to doing something with the Background Tasks extension.