The GPI contacts (1&2) have about 3VDC on them.  These contacts are used to hook up a remote switch or relay to simply turn the amp on and off.  

The GPI contacts (3&4) have no voltage on them.  They are connected to a switch inside the amp that opens and closes depending on whether the amp is on or off.   

These could be connected to an external power source and turn a light on and off for remote amp status.  Or connected to another type of monitoring equipment that can display the status by using the open or closed contacts in the amp. 

Polarity is Pin 1 is (-) pin 2 is (+) but this is irrelevant as you can’t connect anything to this but switch contacts that are unpowered.   

GPI is where you connect an unpowered switch to turn the amp on and off.  

And GPO is where you connect a powered status indicator.