dark horizontal lines on lower portion of built-in retina display after upgrading to Golden Gate on iMac 2021 24 inch

If you are referring to the 24-inch M1 iMac (2021), dark horizontal lines in the lower portion of the built-in Retina display have been reported by other owners of this model. 

Similar reports existed before macOS 27 (“Golden Gate”), so the fact that the lines appeared after the upgrade does not by itself establish that macOS 27 caused the problem.

A useful way to distinguish a macOS-related display problem from a problem involving the internal display system is to check whether the lines are visible in macOS Recovery or during startup. 

On an Apple-silicon Mac, shut down the iMac, hold the power button until the startup options appear, and select Options to enter macOS Recovery. 

Apple provides the procedure here: How to start up from macOS Recovery. 

You can also run Apple Diagnostics by holding Command-D from the startup options; Apple says Diagnostics can help determine whether a hardware component may be at fault: Use Apple Diagnostics to test your Mac.

If the lines remain visible in Recovery or during startup, that would make a problem limited to the normal macOS desktop environment less likely.

If they appear only after macOS loads, software or graphics-related causes would remain worth investigating.

Apple's documentation confirms that the 24-inch M1 iMac has specific display troubleshooting procedures and a dedicated repair manual, including a section for “Display and image issues”: iMac (24-inch, M1, 2021) Repair Manual. 

Apple also provides the complete documentation for the 2021 M1 iMac here: iMac (24-inch, M1, 2021) Manuals and Downloads.

Therefore, the most accurate conclusion is that the lower-display horizontal-line symptom has been reported on 2021 M1 iMacs, but the available information does not establish that macOS 27 caused it or that every affected iMac has the same underlying cause.

Should all the above Tests establish  there is a Hardware Issue 

Have the computer evaluated by the Professionals who have the Special Hardware Software and Expertise to diagnose the issue and offer possible solutions

Make an Appointment at the Apple Genius Bar or call 1 800 MyApple and have them evaluate the computer

Other location Call Centres 

 Contact Apple for support and service

Have you rebooted your Mac? If not, that's the first step. If that does not help, then see if the problem is present when you boot in safe mode. That process performs some system cleanup and clears caches, and also disables 3rd party extensions that may cause problems.

Start up your Mac in safe mode - Apple Support

If that does not correct the issue, test in a new user account. A newly-created user account will have all settings at default, and if the issue does not occur in that new account then it provides guidance on what may need to be changed to correct the issue.

Add a user or group on Mac - Apple Support

Once the testing is complete, the new account can easily be deleted.

Delete a user or group on Mac – Apple Support

Those steps will help to isolate the problem. If the issues occur in safe mode and a new user, then most likely it's a hardware issue and the association with the upgrade is coincidental.

If the problem persists, take a screenshot (Command-Shift-3) and if the issue on the display does not show up in the screenshot, the most likely cause is a hardware problem with the display (cable/panel/etc.). In that case, the next step is to contact Apple for service.

Product Service and Repair Information - Official Apple Support