It seems like you have some misunderstandings of fundamental concepts in Godot. Really, you're not likely to get too far without taking the time to understand the basics. I'd suggest you watch some tutorials to build a solid foundation because debugging your game one error at a time in a single forum question isn't very efficient for anyone involved...
Anyway, I assume you're trying to set the text of your
GoldText node to the value that's passed in as
gold. If that's the case, you want this:
func update_gold_text (gold):
GoldText.text = str(gold)