...
e.g.
Windows 11/ 10/ 8 / 7 / Vista: C:\Users\<current user>\AppData\Roaming\GiD\1516.0\gid.ini
Windows XP: C:\Documents and Settings\<current user>\Application Data\GiD\1516.0\gid.ini
Linux / Mac OS X: $(HOME)/.gid/1516.0/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 Since GiD 15 and GiD 14 developer each GiD version has its own copy of gid.ini in a subfolder.
...
rt 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.