If no devices are being shown on the connections screen and you have followed the troubleshooting steps in this article to ensure your devices are not connected to any other apps, it may be that FulGaz does not have Location Services permissions on your Android device.
To allow Location Services on your device, open the Settings menu on your Android device, then scroll down and open Location. Make sure Use Location is enabled first, then open the App Permission menu below that.
In the App Permission menu, scroll down until you find FulGaz in the list, it will most likely be under the "Denied" grouping. Click on FulGaz.
Ensure that "Allow only while using the app" is checked.
Exit the settings menu, reboot your device (power off / on again) and then restart FulGaz.
FulGaz should now be able to recognise your trainer and sensors.