rfid tag 900mhz

Assuming that an omnidirectional antenna is the optimal solution for a RFID tag, the answer is simple: zero gain.

it is not an omnidirectional antenna. generally the UHF RFID Tags have antennas which are tuned to 900MHZ and are polarized along te lenth of the TAG

Also if the antenna had zero gain, it would mean that the TAG chip would perform equally well without the antenna....

although im not sure it would be in the range of 3-6db

The typical gain of tag antenna is not so low as zero, or so high as 6 dBi. Most tag antennas take the form of dipoles, so their gain is easy to be estimated as that of dipoles, i.e. about 1~2 dBi.