I need to create some space to that I can dual-boot with Windows. Looking at Gparted, it seems that to do that, I need to unmount my /dev/sda1 which contains my Ubuntu system. While searching online, it seems that people were doing it for removable media, but I'm working with one hard drive. Is it safe to unmount to create some space?
You cannot unmount a partition while you are using it. The way to do it is to boot from a live CD or USB drive, and run gparted from there. Then you will usually be able to resize any partitions on your hard drive.
The original Ubuntu disk will work, as will specialized utility disks like gparted magic, but all of them simply use gparted to do the work.
After booting from a LiveCD or LiveUSB, then run Gparted; to unmount the extended partition, FIRST turn off the swap partition (part of extended partition) by right clicking on swap and selecting "swapoff". This should unmount the extended partition at the same time. Was having this problem booting from a LiveUSB, couldn't unmount the extended partition; puzzled, then by accident discovered that unmounting the swap partition would unmount both. After that, was able to resize as I wanted. Hope this helps.