This is the GDScript syntax definition for the Geany text editor / IDE. GDScript is the scripting language for the Godot Game Engine. Geany is a multiplatform text editor or lightweight IDE (more information here).

First copy the file filetypes.GDScript.conf from this repo into the folder ~/.config/geany/filedefs. Then within Geany open the menu "Tools -> Configuration Files -> filetype_extensions.conf" and add the following line into the [Extensions] section:

[Extensions]
...
GDScript=*.gd;
...

And still in the same configuration file modify the [Groups] section as follows:

[Groups]
...
Script=GDScript;
...

After restarting Geany should automatically set the filetype to GDScript when you open a .gd file.

The last step is to tell Godot to use Geany as the external source code editor for GDScript files. In the Godot environment open the Settings menu (in the top-right corner) and select Editor Settings. Here scroll down until you find the options category called External Editor. Simply enter the path to your editor executable, for example /usr/bin/geany, and activate the User External Editor checkbox.

Have fun!