Thanks @David.
I feel that creating a child theme is more of a headache, then using the existing theme.
So I prefer to store my functions.php file whenever I make changes.
And then copy to the new version of GP theme.
I will use Additional CSS for my requirements. I hope that stays the same without any changes.
Even if it changes, I would like to copy that code and save it and copy again.
This way it looks more simple for me.