Ho trovato la risposta tramite questo e questo:
var earthRadius = 6378.0; // km
var rangeInKm = 3000.0; // km
myCollection.EnsureIndex(IndexKeys.GeoSpatial("Location"));
var near =
Query.GT("ExpiresOn", now);
var options = GeoNearOptions
.SetMaxDistance(rangeInKm / earthRadius /* to radians */)
.SetSpherical(true);
var results = myCollection.GeoNear(
near,
request.Longitude, // note the order
request.Latitude, // [lng, lat]
200,
options
);