groebner is the program that actually starts the application program gb. The GRÖBNER library as well as the I/O library expect certain environment variables to be set. Those variables are set in groebner, then mkgroebner is called to check, whether recompilation is necessary, then the application program is called.
Issue the -f option if you want to skip recompilation (of course, only if you are sure that recompilation is superflous!.
Issue the -h option to get information about all command-line parameters that are understood by groebner.