GAP Manual: 27.18. Flat

Flat( list )

Flat returns the list of all elements that are contained in the list list or its sublists. That is, Flat first makes a new empty list new. Then it loops over the elements elm of list. If elm is not a list it is added to new, otherwise Flat appends Flat( elm ) to new.

    gap> Flat( [ 1, [ 2, 3 ], [ [ 1, 2 ], 3 ] ] );
    [ 1, 2, 3, 1, 2, 3 ]
    gap> Flat( [ ] );
    [  ]