IsSimple( G )
IsSimple returns true if G is simple and false otherwise.
A group G is simple if and only if G and the trivial subgroup are the only normal subgroups of G.
gap> s4 := Group( (1,2,3,4), (1,2) );
Group( (1,2,3,4), (1,2) )
gap> IsSimple( DerivedSubgroup( s4 ) );
false
gap> s5 := Group( (1,2,3,4,5), (1,2) );
Group( (1,2,3,4,5), (1,2) )
gap> IsSimple( DerivedSubgroup( s5 ) );
true
GAP 3.4.4