Hello Shahid,
I was reading through the articles and found this
" Unity Gain Antenna.
If a square antenna is visualized and G=1, Ae = lambda^2 / 4*pi. When a dimension is greater than 0.28 lambda (approximately 1/4lambda ) it is known as an electrically large antenna, and the antenna will have a gain greater than one (positive gain when expressed in dB). Conversely, when the dimension is less than 0.28lambda (Approximately 1/4 lambda)(an electrically small antenna), the gain will be less than one (negative gain when expressed in dB). Therefore, a unity gain antenna can be approximated by an aperture that is 1/4lambda x 1/4 lambda.
Think its all about the electric size of an antenna which in the case of rfid is very small hence gain negative.
Regards