That can be confusing because the package is called git-gui but that program is executed in the terminal through the git binary with gui as the parameter (a parameter in the terminal is just a string of text that gets passed to the program (git binary), the string of text in this case being gui. The program will decide what it will do based on that parameter.)
This is what you execute, change directory into your project directory before you invoke the command:
You will also have to initialize a git repository before you can "git gui".
Create a directory where all your source files and other assets will live:
Now you have a local git repo.
The repository is the .git directory in you project directory. dot (.) files & directories are hidden in Linux. You can see hidden files in your file manager by hitting ctrl + h. In the command line you would pass the -a parameter to the ls command:
You may not have git-gui installed. This command will install git, git-gui and a couple of other packages related to git, for Debian distros, use your package manager if you're using another family.
sudo apt-get install git-all
If you already have git installed but not git-gui, run:
sudo apt-get install git-gui
I wrote a bit more information on git than just the git-gui because I thought that people coming to this page are possibly not well versed in Linux and git if they want to use the gui part of it. Sure the gui is good and it really is just an interface to the commands that you will run on the command line. The command line is more powerfúl however because you can do things through it that you can't do through a GUI and it is much quicker. You can learn the gist of git through the gui but I do recommend that you learn about the Linux command line if you want to get serious about programming and computers. There are plenty of books on Amazon that are great for that.