The FXP810 has a peak gain of 1.5dBi at 2.4GHz and efficiencies of 60-70%, increasing to 5dBi and 80-90% along bands 4.9GHz to 6GHz. At 31*31*0.1mm in size this antenna is uniquely valuable for small tag type mobile devices in that it can slip between the battery and the main PCB ground of small devices to get increased performance from the ground coupling effect. Only the top 6.5mm radiating element needs to protrude out from the side of the main board, allowing such devices to have the highest possible performance at smallest possible dimensions, it accomplishes this because it does not need clearance or footprint space on the device board itself that all on-board chip, loop and patch antennas need. Many module manufacturers specify peak gain limits for any antennas that are to be connected to that module. Those peak gain limits are based on free-space conditions. In practice, the peak gain of an antenna tested in free-space can degrade by at least 1 or 2dBi when put inside a device. So ideally you should go for a slightly higher peak gain antenna than mentioned on the module specification to compensate for this effect, giving you better performance.