I am trying to somehow extend the unallocated partition (below sda1) into the sda2 partition to increase its total size. If it is relevant the sda1 is a Windows 10 parition and I'm dual-booting with Antergos (Arch Linux variant). Below is a screenshot from GParted.
Mounted file systems can't be resized. In the screenshot of
gparted, there's a key symbol between /dev/sda2 and ext4; that key symbol indicates /dev/sda2 is mounted. It can't be unmounted while the Arch Linux system on sda2 is running.
Reboot from a liveCD, (or USB equivalent), and run
gpartedfrom that, right click on /dev/sda2 and click 'Resize/Move', etc. Now /dev/sda2 won't be mounted, so it can be resized.
Usually after moving the root directory,
grubshould be updated, or it won't be able to find the system, and Arch wouldn't boot. So after the resize, (and still using the liveCD), open a terminal, do the appropriate
chrootlogin to /dev/sda2, and run
Reboot, remove liveCD. The system should now boot from the newly resized 120GB /dev/sda2 Arch Linux partition.