GAP Manual: 17.1. Unknown

Unknown()
Unknown( n )

Unknown() returns a new unknown value, i.e. the first one that is larger than all unknowns which exist in the actual GAP session.

Unknown( n ) returns the n-th unknown; if it did not exist already, it is created.

    gap> Unknown(); Unknown(2000); Unknown();
    Unknown(97)     # There were created already 96 unknowns.
    Unknown(2000)
    Unknown(2001)