Apple Mainstage

This might be an obvious question but I'm new to the whole vst/daw game. Could I use Mainstage 2 as a plugin in the MPC software? I would mainly want to use it for EQ and synth sounds. Is this possible?

Or is it only for GB and Logic?

No, Mainstage is a plugin host application, you load plugins into it, it doesn't operate at a plugin within other applications.

You could possibly use the MPC plugin within Mainstage, or use the Mainstage plugins within the MPC software, or use some sort of utility to route the audio output of the MPC software into Mainstage.