The antenna gain for 2.4Ghz is 4.5dBi and the antenna gain for the 5Ghz band is 5.5dBi.
The "±1" indicates that there might be a small deviation of up to 1dBi from the specified 4.5dBi / 5.5dBi value depending on the exact frequency and other factors. So for example on 2.4Ghz, the measured value may be a little lower than 4.5dBi or a little higher.