I am not sure, but it seems like the Gamer Edition has certain pre-loaded software and features that make for a better gaming experience. Whereas if you installed Manjaro KDE Plasma, it would include the base programs that Manjaro chooses.
If you do want to switch over, you can back up your data and do a new install via a USB flash drive. Here’s a good place to read up on how to do so: I don't know where to start! - Your Linux Introduction