The equalizer we get through the settings menu of Google Play music is nothing but the re-skinned built in EQ found in older android phones. It has 5 bands, bass and stereo widening controls.
And as @user1524241892777 mentioned "Equalizer exist independently not attached to any app. Such Equalizer affects not only voice coming from a music player but also from YouTube or any video player too. E.g. Dolby Atmos..."
Try setting up the EQ inside music player, listen to the sound. Now open youtube, vlc or any other music player. You won't notice the effect of the EQ.
The only other player which can make use of this EQ inside google play music is PowerAmp. That player has option to enable the EQ available inside google play music. Otherwise this is not an independent EQ.
Independent EQ affect system wide audio.
However having an independent EQ has its own pros and cons as well.