To setup what type of equalizer you want, go to the equalizer preferences (Preferences -> Audio -> Equalizer)
Number of bands: Choose anywhere from 2-10 bands. A band represents a range of frequencies that you can adjust the gain for. Each band will add more cpu processing, so only choose the amount you need. 5 is a good number to start with.
Quality: GMMP includes 3 different DSP filters that can be used by the equalizer. To simplify things, each filter is associated with a different quality setting.
-Default (Recommended): This is a RBJ Bi-quad filter. The quality is good, but it has the worst freqency response out of the 3.
-High: This is a 1st-Order Butterworth filter. This has a much better frequency response, so gain adjustments to one band will have less effect on adjacent bands.
-Highest: This is a 2nd-Order Butterworth filter which has a very good frequency response. This is very CPU intensive (around 6+ bands on an HTC thunderbolt results in the audio skipping)
The Equalizer View:
The quick nav bar should now contain and equalizer button. Click on that to get to the equalizer view pictured below.
Pressing the menu button will bring up a bunch of equalizer specific options.
Toggle EQ: Toggles the equalizer on and off
Save Preset: This will save the values of pregain and each band to a preset. Simply provide it a name and it will show up in the preset spinner. Note: A preset is specific to the number of bands, so you cannot use a 10 band preset on a 5 band EQ.
Delete Preset: Will bring up a list of all the presets. Chose whichever you would like to delete
This post will be updated as more equalizer features are added.