Hi, I wanted to know if the passage theme will support Gutenberg or currently does?
And will it simply be a plugin update or theme update?
From my quick research, this is still in beta - https://wordpress.org/news/2018/11/wordpress-5-0-beta-3/
The biggest worry for people is Visual Composer's support/handle of this, since they have overlapping functionality. Here is an open thread regarding VC's part in this - https://themeisland.ticksy.com/ticket/1657233
I have not personally tested the beta release with this theme, but as a webmaster, you choose when your site gets updated to the latest version of wordpress. I'd suggest, if you are concerned, wait for it to go live (officially), and see if others are having issues. Also, as best practice, you should have a staging site to test these kinds of major updates.
To be clear, I have no control over how WP handles their updates, and if it affect this theme, it will affect thousands of sites, since (1) VC is the #1 WP page builder plugin of all time, and (2) this theme follows all of the latest standards required by WP and Themeforest (which is really strict).