It can work for GDScript or any other type of text file, however it was not designed to be used as a code editor. You will have a better time with the built in editor. Jotson made a thing inspired by textreme for the built in text editor but it is very different in style. https://github.com/jotson/ridiculous_coding
It was made by taking Godots built-in TextEdit node(the same one used for the built-in code editor) and modifying it until it was unrecognizable. Figuring out where to place the effects is a whole different can of worms. I can explain how a specific part of it was made in detail if you are interested!