Some of the earlier games in the pack used drag and drop (back when I was still learning the basics) but most of them are written entirely in GML, especially the newer ones. I can't say there's any one single way to learn GML. Mostly if I wanted to do something and I couldn't figure it out I just looked up online tutorials and the answer stuck with me. I guess I'd advise the following:
- Look for D&D to GML converters (here's one on itch) and try getting into the habit of replacing simple D&D actions with the relevant line of code.
- Look for and download a few very simple demo projects written in GML. Some come with GameMaker already, some are available on the Marketplace. A lot of demos are well commented, so it should help you learn the code.
- Set yourself a few simple tasks to learn before diving into any complicated game mechanics. A lot of the tasks required for puzzle games make good learning tools. For example a "lights out" style game is quite easy to make in GML.
Hope that helps!