The current GAMS version on Unix (that includes the Mac) does not work in directories with names that contain a space. The actual file name also cannot contain a space. GAMS itself produces the (meaningful) error message:
$ cd "My Directory" && gams mymodel.gms *** Error: White space in CurrentDir CurrentDir = /Users/michael/My Directory
$ gams "my model.gms" gams: **** Error: Parameter error(s) : Reading parameter(s) from "command line" : *** Error unknown .suffix ".gms" for key "model" : Finished reading from "command line"
If you are using GAMS Studio, you will get one of the error messages described above in the Output window but even before Studio will give you a pop-up similar to this:
As a side note, it is not a good idea to run GAMS in a synchronized folder like Google Drive. Here you can read more about this topic.
The remedy for this error is clear. Avoid file and path names with spaces. If your user name already contains spaces you will have to run GAMS in a folder outside your user directory. If you have admin rights you can create such a directory easily enough. Without admin privileges you can create a subdirectory under
/tmp and do your GAMS work there.
Help is on the way. The upcoming 25.2 (expected December 2018) will allow spaces in path and file names not only for Windows but also for UNIX (and Mac).