An antenna usually has a gain it is expected to have at its operating frequency, and a useable bandwidth determined by some minimum gain. This is often 3 dB lower than the peak gain. In this case the gain bandwidth would be the bandwidth over which the gain is within 3 dB of the peak. Alternatively, the gain bandwidth is the bandwidth for which the gain is higher than some set level.
In your figure it is not possible to determine the 3 dB bandwidth because measurements are too far apart in frequency, and both the measurements on either side of the peak gain are more than 3dB below the peak.