Ubuntu – GParted – Cannot open display

The trick here is to use an Xserver on another machine to display the GUI elements you can't see on a server. Set the environment variable to the local machine running the Xserver. Where this is an IP address and the screen # on the X server.

export DISPLAY="192.168.1.5:0"

then run :

sudo gparted 

With another Ubuntu machine you will need to change firewall rules to allow this. Using windows you can do this with MobaXterm which runs a local X server.

A better way to understand disk space usage is to use the handy app Disk Usage Analyzer, or to use df -h and du -h, which will show you disk usage by different volumes and files. The main drives you are concerned about are those beginning with /dev/sd. These are physical drives in your system (or partitions if they have a number at the end).

If you would like to "extend your disk space" to another drive, you can look at RAID (to increase the size of your entire root filesystem, ans share the used space among both drives, allowing you to expand every folder). You could also simply plug in another hard drive and use it as such. For example, just store all your big files or python projects there (it would be mounted in /media by default, but this can be changed; see link below.). Another solution would be to move your home folder to another drive. Copy your files to another drive, and then set the other drive to mount in /home/username (see https://www.man7.org/linux/man-pages/man5/fstab.5.html). This would put everything in ~ (Desktop, Downloads, Documents, etc) on your second hard drive.

EDIT For understanding loop devices, see here.

EDIT 2 The output you added from df -h shows that you have two separate partitions: one for /, and one for /home. You have plenty of disk space available in /home, which contains your Desktop, Documents, etc. You may be able to shrink this partition and allocate more to the rest of the system, but that would be a different question (that has probably already been answered).