GMEdit

GMEdit is a high-end, open-source code editor for GameMaker.

It represents what I consider to be the most important when working with code - being able to edit code quickly and comfortably, with features expected from a modern day code editor and conventional tabbed document design.

Rough lineup of features:

• Supports a variety of versions, including GameMaker: Studio, GameMaker Studio 2 (pre-2.3 and 2.3 formats), and limited support for legacy (GameMaker≤8.1) projects.
  It can also be used to edit code for GML-based mods for games like Nuclear Throne or Rivals of Aether.
• Spots a high-performance code editor (Ace), extended and fine-tuned for GML.
  Comes with GameMaker-styled keyboard shortucts that can be customized.
• Has combined editors for objects, timelines, and extensions, allowing to view/edit multiple events/moments/scripts at once.
• Fast save and load operations; only changes files it needs to.
• Has a variety of syntax extensions to ease writing repetetive bits of code.
  Changes are non-destructive and the code remains readable/editable in base IDE.
• Has custom theme and plugin support.
• Free and open-source.

Overall, it can be viewed as a more pleasant alternative to GameMaker's base IDE, and becomes increasingly more advantageous the more code you write or the more complex your code gets.

By design it is something that you run alongside the base IDE, but there are ways you can avoid switching back and forth to run the game.