A gain equalizer for your frequency range is easily made as:
- a series capacitor which has its reactance falling with frequency; by using such series capacitor, amplifier gain will be reduced at lower frequency, and increased at higher frequency.
- a parallel inductance which has its reactance rising with frequency.
The combination of the two above forms a high-pass filter; you can use one or both, and calculate the response from the "base impedance" of 50 Ohms.
If you need to reduce amplifier gain in proportion with frequency, use the reverse: a parallel capacitor, and/or series inductance.
To equalize a gain response in an existing amplifier, use capacitor trimmers to adjust the response as desired, best on a test setup with a sweep generator and detector plus an oscilloscope.