Negative gain in CST antenna design

Hello,
1.Can anyone help me regarding the gain parameter in CST Microwave Studio? How do we check it using the radiation pattern?
2. When I plot the maximum gain vs frequency graph when I use a 42mmx28mm patch (85 mm x 85 mm ground plane) I get a positive gain. But as soon as I change it to a 60 mm x 60 mm patch (90 mm x 90 mm ground plane) the gain starts becoming negative (-3 dB to -7 dB) at the resonant frequencies.

Thanks in advance.