The nice thing about Geometer is that you can use the two in combination---do most of the work with the graphical user interface, and then submerge yourself in the textual description to fine-tune your diagrams.
Geometer has a little editor built into it, and when you issue the Edit Geometry command (or with speed key Ctrl-e), it fires up this little editor on the textual description of the currently viewed diagram. The advantage of using Geometer's editor is that when you finish your editing, Geometer immediately tries to reload the file, and if there are errors, you will be returned to the editor with the offending line highlighted.
You're welcome to use your favorite text editor as well (just make sure you save the files in "text" or "ascii" format), but then each time you write the file, you'll have to go to Geometer and use the ReOpen command. Then if there are errors, you'll have to go back to your editor, find the right line, fix it, re-save, and so on. Most Geometer files are tiny---a 100 line file would be quite large---so you probably don't need the power of a full-blown Emacs or equivalent.
Geometer's editor is a basic cut-and-paste editor, but it does have a couple of Emacs-type commands that you can learn about in the reference section on the editor. But the files are usually so small that you won't need to worry about this.
Between the time you fire up the editor with the Edit Geometry command and you exit (using the Save command in the editor's pull-down menu or the Ctrl-s keyboard shortcut), you can only edit the text. The current diagram visible in the Geometer window is frozen.
File Format
Names of Primitives
Layers
Text
Numerical Manipulation
Transformations
Macro Definitions
Scripts
More on Colors
Primitive Types
Command List
Text Editor
"Secret" Keyboard Commands
Startup Options