I need to know 2 things: (1) which version of GTK is included with Z15.3 and (2) if not GTK 3.24, I need to know how to upgrade to that. Thank you.
Zorin OS 15.3 comes with gtk 3.22.30 by default. You might be better served to install Zorin OS 16, which comes with gtk 3.24.20 by default.
Zorin OS 16 is currently a Beta:
The non-Beta release may be a week away or so. (As of 06 Aug 2021.)