GAP Manual: 45.10. Unbind

Unbind( rec.name )
Unbind( list[n] )

In the first form Unbind deletes the component with the name name in the record rec. That is, after execution of Unbind, rec no longer has a record component with this name. Note that it is not an error to unbind a nonexisting record component. rec must evaluate to a record, otherwise an error is signalled.

In the second form Unbind deletes the element at the position n in the list list. That is, after execution of Unbind, list no longer has an assigned value at the position n. Note that it is not an error to unbind a nonexisting list element. list must evaluate to a list, otherwise an error is signalled.

    gap> r := rec( a := 1, b := 2 );;
    gap> Unbind( r.a );  r;
    rec(
      b := 2 )
    gap> Unbind( r.c );  r;
    rec(
      b := 2 )
    gap> l := [  , 2, 3,  5,  , 7,  ,  ,  , 11 ];;
    gap> Unbind( l[3] );  l;
    [ , 2,, 5,, 7,,,, 11 ]
    gap> Unbind( l[4] );  l;
    [ , 2,,,, 7,,,, 11 ] 

Note that Unbind does not evaluate its argument, otherwise there would be no way for Unbind to tell which component to remove in which record, because it would only receive the value of this component.