If all you want is a new race name, then it should be sufficient to add a new entry to races.gd. Having appropriate stats is more important to gameplay than the code. Though you will not be able to add new descriptions for the race by only editing races.gd, you would have to make corresponding changes to description.gd.
As far as tutorials, I'd recommend taking a Python langauge coding tutorial, there are plenty of sites that offer interactive tutorials online for free, just google it. GDScript is based on Python. This page covers the basics of GDScript, which will make more sense if you know the basics of Python: https://docs.godotengine.org/en/stable/getting_started/scripting/gdscript/gdscri...
There are no tutorials for making the specific types of changes to Strive that you wish to make. Though if you wanted to distribute the changes as a mod there is a tutorial: https://strive4power.fandom.com/wiki/Modding_Guide
