A Gain Equalizer is a special attenuator that has a frequency response that is intentionally not flat. There are many reasons you might want to use an equalizer. Perhaps the most common is to fix the problem of wide-band microwave systems. Most stuff has a negative gain slope (it has less gain, or more loss, the higher in frequency you go) This includes not only amplifiers, but passive stuff like coax cables and micro-strip transmission lines as well. The antidote to negative gain slope is adding a linear gain equalizer that has a positive gain slope to your lineup.