The GAMS system directory is the directory, where the GAMS System has been installed. Under Windows this is by default
C:\Program Files\GAMSX.Y where
X.Y indicates the distribution (i.e 22.4).
If GAMS has been installed to another directory and you don't know how to locate this just run this tiny GAMS program:
The listing (lst) file contains the requested information:
---- 1 c:\gams\