ArchiCAD offers an integrated development environment (IDE) for easy access to the object's source code. Each part of an object  - such as the different scripts, binary stuff and pictures are concentrated in one editor panel. You edit via the file menu item "Open Object".

But these built-in editing functions have a number of shortcomings and you need to be running a full license version of  ArchiCAD.

There were two main external "full featured" editors developed out there (Black Turtle and 3nF Scripter), but both have stopped development and neither will recognize GDL syntax for versions more recent than about ArchiCAD 7. They are no longer recommended.

You can find links to further GDL development tools on this page: www.gotogdl.net

You can edit script text in a simple text editor too, but you have to copy and paste all text from the IDE to the editor and back. This only makes  sense if you are editing large projects or want to anaylse exisiting scripts with the extended text editing features of the external software.

I recommend Proton (download) for which I offer an extension for syntax highlighting. This has to be copied into the syntax scheme folder of the proton program folder. It  automatically highlights commands, variables, Text, comments and global and optional parameters in different colours and/or styles.  Extension download see below.