@Matty-Lad,
It depends on how automatic you want it.
If you are willing to open Notepad++, open the file, and run a macro to do a replacement, that can be done with native Notepad++ (see below).
If you want to have notepad++ already open, but have it automatically open the file, you cannot do that in a macro; however, you could install a scripting plugin like PythonScript, and do something that automatically opens the file, runs the replacement, saves, and closes the file again.
If you choose to go the macro route, you can setup the Macro using the following sequence:
Macro > Record Macro
Search > Replace
FIND WHAT = "sparse_infill_pattern": "gyroid"
REPLACE WITH = "sparse_infill_pattern": "gyroid"
SEARCH MODE = Normal
REPLACE ALL
File > Save
Macro > Stop Recording
Macro > Save current recorded macro
Give it a name, like BambuGyroid
Optionally, give it a keystroke that you can remember
OK
From now on, using Macro > BambuGyroid (or the keystroke you chose) will run that specific replacement and save.