NOTE on avatars: you need a gravatar for that:
https://en.gravatar.com
1. Will the code added to the functions.php file cause problems due to GP updates? Like functional duplication?
Code for adding theme_support options won’t cause a conflict.
2. Will the code added to the functions.php file cause problems due to updates to the Gutenberg editor?
We can’t say anything for certain here, but generally if WP releases a function and they make a change in the future then they will ensure backward compatibility. So again this should not be an issue.
3. Plugins vs Child Theme.
Plugins always add some productivity value and make it easier to maintain.
Performance wise between plugins and child theme, there should be no difference.