GSX requires at minimum 2 parking spots, one for your airplane and at least another one to spawn service vehicles not tied to a parking spot ( fuel, catering, deice and follow me ), that airport has just 1 parking spot so, once you take it, there's no place to create those extra vehicles.
We'll try to detect this situation and issue a clear error message about this, but surely this airport can't be used, unless some extra parking spaces will be added.