gain its a measure you have to perform where the radiated fields of your structure are strong, having this in mind, the radiated fields of most microstrip patch antennas happen broad side ( normal to the patch), in HFSS you plot gain in dB ( with out normalizing), if your antenna has 0dB it means that it doesn't have any gain in that particular direction, usually you need 0dB gain in all directions for a wrap around antenna, plus G= efficiency x Directivity, so either you have a low directional antenna, or you have a high loss antenna .