GoogleMapsXF.xaml

<google:Map MyLocationEnabled="True" ItemsSource="{Binding Items}">
  <google:Map.ItemTemplate>
    <DataTemplate>
      <google:Pin 
        Type="Place" 
        Position="{Binding Position}" 
        Label="{Binding Name}" 
        Address="{Binding FullAddress}" 
        Icon="{Binding Pin, Converter={StaticResource IconBitmapConverter}}">
      </google:Pin>
    </DataTemplate>
  </google:Map.ItemTemplate>
</google:Map>

IconBitmapConverter.cs

using System;
using System.Globalization;
using Xamarin.Forms;
using Xamarin.Forms.GoogleMaps;

namespace YourNameSpace.Converters
{
    public class IconBitmapConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (!(value is string icon)) return null;
            return BitmapDescriptorFactory.FromBundle(icon);
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }
}