Difficulty: ★☆☆☆☆
In case you are coming from Ubuntu and wondering why there is no fractional scaling option on Manjaro GNOME with Xorg, well here is the answer. 
Fractional scaling on GNOME - Xorg is not supported by the project, they do officially support that functionality only on Wayland.
However, Ubuntu has patched some GNOME components to make it work on Xorg and thanks to their effort, we can use some of that on Manjaro.
Before installing please keep in mind that fractional scaling on Xorg is a experimental feature!
Installation:
Simply install mutter-x11-scaling from the repository and reboot.
Now you can enable/disable fractional (per monitor) scaling via gsettings.
To enable fractional scaling after installation run:
gsettings set org.gnome.mutter experimental-features "['x11-randr-fractional-scaling']"
To disable fractional scaling run:
gsettings reset org.gnome.mutter experimental-features
More information can be found on Github.
Note:
With GNOME 43 the gnome-control-center-x11-scaling package is back, which includes patches for proper multi-monitor scaling and a toggle to enable/disable fractional scaling.
If you install this package additionally, the above mentioned instruction via gsettings can be ignored.
