There are two popular ways to check disk usage in Linux terminal du command and df command. The du command is more for checking the space used by a directory and the df command gives you the disk utilization on filesystem level.
There are more friendly ways to see the disk usage in Linux with graphical tools like GNOME Disks. If you are confined to the terminal you can use a TUI tool like ncdu to get the disk usage information with a sort of graphical touch.
Gdu is such a tool written in Go hence the 8216g8217 in gdu. Gdu developer has benchmark tests to show that it is quite fast for disk usage checking specifically on SSDs. In fact gdu is intended primarily for SSDs though it can work for HDD as well.
If you use the gdu command without any options it shows the disk usage for the current directory you are in.