It's good you're taking advantage of the Mileage feature in QuickBooks Online. I'll be glad to steer you in the right direction by sharing more info on this.
To answer your question, the feature does use the GPS on your cell phone. You'll just need to make sure your phone has correct location permissions enabled. If you're using iOS/Android, it's important to take note of the following:
iOS
Make sure your phone’s location permissions are set to “Always”.
For the most accurate trip tracking, make sure to enable Motion & Fitness permission.
Android (Coming soon)
Make sure your phone’s location permissions are set to “Allow all the time” and physical activity permission to “Allow”.
To start tracking your mileage, you’ll need to turn it on in the QuickBooks Mobile app.