project file GPS, and for Source_List_File use "xxx.txt";

[thymallus1@free.fr]

I have a project file RTS3-GW.GPR which says:
project RTS3_GW is
   for Source_Dirs use ("C:\(E)\Ada\Gnat\RTS3-GW\Sources");
   for Source_List_File use "(C:\(E)\Ada\Gnat\RTS3-GW\Sources\gw-source_files.txt");
       [...]
end RTS3_GW;

Environment Variables are set like this:,
C:\(E)\Ada\Gnat\RTS3-GW\Sources is at the beginning of ADA_INCLUDE_PATH.
RTS3_GW.GPR is in GPR_PROJECT_PATH.
The list file gw-source_files.txt is in C:\(E)\Ada\Gnat\RTS3-GW\Sources.

If in GPS I launch [Build] for the project there is the message:
[2014-01-22 16:53:52] C:\(E)\Ada\Gnat\RTS3-GW\Sources\RTS3_GW.gpr:6:29: file with sources "(C:\(E)\Ada\Gnat\RTS3-GW\Sources\gw-source_files.txt"  does not exist
followed by 131 warnings (1 for each file in the list): "source file "xxx.adb" (or "xxx.ads") not found.

Of course all 131 files are present in the Source directory.
So it is not possible to produce an executable. 
I do not see why the source file list cannot be found by GPS.  C:\(E)\ is a valid directory name in Windows 7. So what?