MainPageViewModel.cs

    public class MainPageViewModel : ViewModelBase
    {
        private readonly IDisposable _gpsDisposer;
        ...
        
        public MainPageViewModel(INavigationService navigationService, IGpsManager gpsManager)
            : base(navigationService)
        {
            ...
            
            // Vamos inscrever a viewmodel para ser
            // notificada quando houver uma nova leitura
            _gpsDisposer = gpsManager.WhenReading()
                .Subscribe(OnNext, OnError);
        }

        public void OnError(Exception error)
        {
            CurrentLocation = error.Message;
        }

        public void OnNext(IGpsReading reading)
        {
            CurrentLocation = $"Latitude: {reading.Position.Latitude} | Longitude: {reading.Position.Longitude}";
        }

        ...

        public override void Destroy()
        {
            base.Destroy();
            // Lembrando sempre de desinscrever o observador
            _gpsDisposer?.Dispose();
        }
    }