is it possible to create an App with Flutter, to create connection with two smartphones? And then to get the GPS coordinates and calculate the distance between smartphones, the maximum distance will be 100-200 meters
You could try using Wifi-Direct with a specific (your application specific) SSID. Here's a link to a guide on how to do so in android https://developer.android.com/training/connect-devices-wirelessly/wifi-direct You'll need to request the necessary permissions though Bluetooth is another option for peer to peer communication on smartphones but I think that the range limitation might rule it out