post equalising technique &opamp circuit
The gain is zero dB. That means unity gain. The dB is used to express relative signal and power strength. If the gain was 6dB, the amplifier would have a gain of 2. If the gain was -6dB, the gain would be 0.5.

At the higher frequencies the small capacitor's Reactance has reduced to the point where it does influence the gain by starting to take current away from the inverting input. The opamp action at these high frequencies is to raise its output level to compensate: -



As frequency rises even higher the op-amp's bandwidth limit is reached and gain falls away - note that on the right-hand side of your graphs the gain has gone below 0dB and this means the gain has fallen below unity (and will continue to fall).

If you use the formula: -

H(f) = \$\dfrac{Sin(\dfrac{\pi f}{F_S})}{\dfrac{\pi f}{F_S}}\$

For Fs = 125 MHz and f = 40 MHz, H(f) in decibels is 20 log\$_{10}\$(0.8398) = -1.52dB

Looking at your top graph, it seems the op-amp is over-compensating (about +2.5dB at 40 MHz) so try reducing R1 to 200 ohms or 180 ohms.