GMK - hpgDesigns/hpgdesigns-dev.io GitHub Wiki
The GMK format was used in GameMaker version 5.3a to 8.1 and was discontinued (although still supported for import and export) in GameMaker: Studio. This article outlines the API developed by Zach Reedy for NaturalGM.
The GMK API is written in C++ and is cross platform across all devices. All API is kept in the Gmk namespace.
This is the main API class.