Derek Holt asks how GAP should be cited. I think the best way to cite
a program system and also the most often used one is to quote its
manual:

[S+ 92] Martin Sch"onert et.al. GAP -- Groups, Algorithms, and Programming.
Lehrstuhl D f"ur Mathematik, Rheinisch Westf"alische Technische
Hochschule, Aachen, Germany, first edition, 1992.

So a BibTeX entry for a quotation of the GAP manual should read:

@string{ RWTH      = "Rheinisch Westf{\accent127 a}lische Technische
                      Hoch\-schule"                                     }
@string{ RWTHLDFM  = "Lehrstuhl D f{\accent127 u}r Mathematik,
                      Rheinisch Westf{\accent127 a}lische Technische
                      Hoch\-schule"                                     }
@string{ RWTH-A    = "Aachen, Germany"                                  }

@manual{Sch92,
author        = "Martin Sch{\accent127 o}nert and others",
title         = "{GAP} -- {Groups}, {Algorithms}, and {Programming}",
year          = "1992",
edition       = "first",
organization  = RWTHLDFM,
address       = RWTH-A,
notes         = "PAGES: 700",
keywords      = "groups; *; gap; manual"
}

If you are not using BibTeX, here is the bibliography entry produced by
BibTeX. You can use this inside the bibliography environment of LaTeX.

\newcommand{\etalchar}[1]{$^{#1}$}

\bibitem[S{\etalchar{+}}92]{Sch92}
          Martin Sch{\accent127 o}nert et~al.
\newblock {\em {GAP} -- {Groups}, {Algorithms}, and {Programming}}.
\newblock Lehrstuhl D f{\accent127 u}r Mathematik,
          Rheinisch Westf{\accent127 a}lische Technische Hoch\-schule,
          Aachen, Germany, first edition, 1992.

Joachim Neubueser