The package can be downloaded from www.icm.tu-bs.de/ beick/soft/sglppow/sglppow-Version.tar
Then tar -zxvf slgppow.tar produces a directory sglppow. This should be moved into the pkg directory of a GAP installation.
The package is set up so that after loading it into GAP with LoadPackage the full functionality of the package is available.
sglppow manual