If your system is not up-to-date, installing packages without updating the rest can lead to breakage, because the package you wish to install was built against the newer versions of the shared libraries on your system.
Partial upgrades are not supported, even though it is possible to still install software from the repositories, as @XRaTiX has explained ─ it’s even easier from the command line. 
sudo pacman -S gparted
Manjaro is a curated rolling-release distribution, which means that you must keep your system up-to-date, but it also means that the frequency by which new packages are pushed out is a lot lower than with non-curated rolling-release distributions such as Arch, Gentoo, et al.
If you cannot keep your system up-to-date, then I’m afraid Manjaro isn’t the right distribution for you, and then you’d probably be better off with a fixed-point-release distribution ─ e.g. Mint, Ubuntu, Debian, CentOS, Mageia, openSUSE Leap, et al.