GAP Manual: 10.18. NextPrimeInt

NextPrimeInt( n )

NextPrimeInt returns the smallest prime which is strictly larger than the integer n.

Note that NextPrimeInt uses a probable-primality test (see IsPrimeInt).

    gap> NextPrimeInt( 541 );
    547
    gap> NextPrimeInt( -1 );
    2