Why GPS not Working when Gear S3 is Paired with a Phone?

The Gear has a GPS sensor so that you can check your real-time location information without connecting to a mobile device. When Gear and mobile device are connected, this feature uses mobile device’s location information. Activate the mobile device’s location feature to use the GPS sensor.

When you use the Gear while it is not connected to a mobile device or when you use an app such as Alti-Barometer, this feature uses your Gear’s location information.

To Enable Gear's GPS, Navigate to Apps screen > Tap Settings > Connections > Tap Location to Activate

Pictorial representation of the above settings are as follows :