Launch regedit in Windows, find the uninstall key for that Opera GX and delete it. You should be able to find it as a subkey under "HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall" or as a subkey under "HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall". You can look in the right-hand pane for the the version of Opera so you know what one to delete.

That will get rid of the uninstall entry for you. Then, you can manually delete Opera's install folder and then reinstall it if you want. See https://www.reddit.com/r/operabrowser/wiki/opera/clean_opera_from_windows for more details.