In the same vein as VST and Audio Unit plugins, GMPI is a plugin API for Software Instruments and Effects.
The GMPI project was started to gather the best parts of existing specifications and bring them together into an easy-to-use, open, powerful, and cross-platform alternative to the proprietary standards offered by Steinberg, Apple, and other vendors.
GMPI was instigated by the MMA (MIDI Manufacturers Association) as a collaborative effort. This implementation of GMPI is not endorsed by the MMA, but we’ve endeavored to adhere to the specification as closely as is practical.
there’s an in depth FAQ