CompleteSubgraphs( gamma ) 
CompleteSubgraphs( gamma, k ) 
CompleteSubgraphs( gamma, k, alls )
This function returns a set K of complete subgraphs of gamma, which must be a simple graph. A complete subgraph is represented by its vertex set. If <k> > -1 then the elements of K each have size k, otherwise the elements of K represent maximal complete subgraphs of gamma. The default for k is -1, i.e. maximal complete subgraphs.
The   optional  boolean  parameter  alls  controls  how  many  complete
subgraphs are returned.  If alls is true (the default), then K will
contain (perhaps properly) a  set of  gamma.group orbit-representatives
of  the size k  (if <k>  > -1) or  maximal (if  <k>  < 0) complete
subgraphs of gamma.
If alls is false then K will contain at most one element.   In this
case, if <k> < 0 then K  will  contain  just  one  maximal  complete
subgraph, and if <k> > -1 then K will contain  a complete subgraph of
size k if and only if such a subgraph is contained in gamma.
    gap> gamma := JohnsonGraph(5,2);;
    gap> CompleteSubgraphs(gamma);
    [ [ 1, 2, 3, 4 ], [ 1, 2, 5 ] ]
    gap> CompleteSubgraphs(gamma,2,false);
    [ [ 1, 2 ] ] 
GAP 3.4.4