I don't run it myself, but loaded it on my test vm and yeah the GUI does not seem to launch.
Tried from the cmd line and here is the error...
opensnitch-ui
Traceback (most recent call last):
File "/usr/bin/opensnitch-ui", line 41, in <module>
from opensnitch.service import UIService
File "/usr/lib/python3.12/site-packages/opensnitch/service.py", line 15, in <module>
import opensnitch.proto as proto
File "/usr/lib/python3.12/site-packages/opensnitch/proto/__init__.py", line 19, in <module>
from packaging.version import Version
ModuleNotFoundError: No module named 'packaging'