The result of ls command will depend of umask.
g+s will set sgid to the file. Check here for more deep information about SUID SGID
so if your umask for example is 044022 the result will be something like:
-rw-r--r-- 1 romeo UsersGrp canard 0 Jan 31 20:58 deux -rw-r-Sr-- 1 romeo canard UsersGrp 0 Jan 31 20:58 un