This is mandatory reading
When you have familiarized yourself with how updates or more correctly syncronizing work in the Arch/Manjaro eco system - you will know that you can use the following command on a fully synced system to install any package you like.
sudo pacman -S pkgname
That also means you can use the command on a system which is not sync’ed.
Doing so is highly discouraged, as the system when not sync’ed, will be in - what Arch call - a partially sync’ed state - and therefore unsupported.
If you get issues with system instability or weird system behavior - you know why - do not ask the forum for help - because you are in the mess by your own choice.
If you are not using KDE but have installed Gnome using the package manager then you should uninstall the KDE system (be careful though - dependencies) using the package manager.
This will prevent updating packages which is no longer used and thus can be removed.