Menu: Files->Save...
Toolbar: 
Using the Save command saves all the information relating to a project - geometry, conditions, materials, mesh, etc. - to the disk.
When a project is saved, GiD creates a directory with the project name and the extension .gid. Files containing all the information are written to this directory. Some of these files are binary and others are ASCII. You can then work with this project directory as if it were a file.
You do not need to write the .gid extension because it will automatically be added to the directory name.
Caution: Be careful if changing some files manually in the Project.gid directory. If done in this way, some information may be corrupted.
Advice: It is recommended to save the project at regular intervals so as not to lose any important information. It is possible to back up files automatically by selecting this option in the Preferences menu (see Preferences).