I had one of these laying around I had used on my last 2 GL1800s and used it to get the factory filter off so if you're looking for a nice, tough, compact filter wrench for $10 ish, not too bad.
http://wingstuff.com/pgroup_detail/359_oil_filters_wrench/2065_goldwing_oil_filter_wrench_fits_gl1500_gl1800/?goto=%2Fpgroup_list%2Fgl1800%2F359_goldwing_oil_filters_wrench%2Fdes%2F
http://wingstuff.com/pgroup_detail/359_oil_filters_wrench/2065_goldwing_oil_filter_wrench_fits_gl1500_gl1800/?goto=%2Fpgroup_list%2Fgl1800%2F359_goldwing_oil_filters_wrench%2Fdes%2F