Each user has its own copy of some GiD settings (like preferences variables, or window sizes).
This settings are automatically saved and loaded.
This information is saved in a file named 'gid.ini' that is stored in a different place depending on the platform and the GiD version
Windows 10/ 8 / 7 / Vista: C:\Users\<current user>\AppData\Roaming\GiD\15.0\gid.ini
Windows XP: C:\Documents and Settings\<current user>\Application Data\GiD\15.0\gid.ini
Linux / Mac OS X: $(HOME)/.gid/15.0/gid.ini
And by default some Windows folders like "AppData" are hidden. (and for Linux filenames starting by . are hidden)
Note: In GiD 14 official versions in Linux/macOs the file name was '.gidDefaults' and is directly at $(HOME)
Note: In GiD 15 and GiD 14 developer each GiD version has its own copy of gid.ini in a subfolder.
Reset user preferences
The easiest way to delete this user settings file, without need to find it on disk, is to start once as 'Repair GiD'
and then a window will appear: set the 'Reset preferences' checkbox and press Ok
If the user defaults is deleted the next time that GiD start it will use its own default values.