GAP Manual: 27.36. Iterated

Iterated( list, f )

Iterated returns the result of the iterated application of the function f, which must take two arguments, to the elements of list. More precisely Iterated returns the result of the following application, f(..f( f( list[1], list[2] ), list[3] ),..,list[n] ).

    gap> Iterated( [ 126, 66, 105 ], Gcd );
    3