Maybe it's a silly question but couldn't find an answer for it other than for the opposite question.
If I have my website already connected with Google Analytics, do I still need to add gtag.js in order for UTM properties in an URL to be tracked?
I'm not really sure how would Google be able to track it otherwise.
You have to add the Google Analytics snippet in all pages. So, in this case if UTM parameter are in the URL it will be tracked automatically.