I want to edit the hex data of a file in Godot, I know C++ is better suited for this, but I was just wondering whether this is a feature in GDscript.

I'm not sure what you mean by "hex data".

The FileAccess method get_buffer() reads raw data from a file, and the method store_buffer() writes it. The seek() and seek_end() methods change the position of the file reading/writing cursor. (FileAccess is in Godot 4. In Godot 3, it's named File.)
https://docs.godotengine.org/en/4.1/classes/class_fileaccess.html

So you can read the data, modify it, and write it back to the file.