At runtime means not in the Spine editor, but in your application. The Spine Runtimes, which you linked to, are the officially supported runtime implementations for many langauges and game toolkits. The documentation for it is here:
Spine Runtimes Guide
GMS is a game toolkit which integrates the Spine Runtimes to load, manipulate, and render Spine animations. GMS should provide you an API to manipulate bones and other parts of the animation data. Ideally they provide you direct access to the Spine Runtimes APIs, else maybe they provide a wrapped API that has most or all of the same functionality.
One you know what GMS provides for manipulating your skeleton, you should have better idea of how to do what you need. I expect what you want is to prevent the foot getting too close to the root of the leg, but it isn't clear to me how you want it to behave. You can't just say, "I want to be able to move the foot anywhere and have no problems", you'll need to decide what you want to happen to avoid such problems.
