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
GAP 3.4.4