GAP Manual: 4.10. Size

Size( D )

Size returns the size of the domain D. If D is infinite, Size returns the string "infinity". D may also be a proper set (see Set), in which case the result is the length of this list. Size will, however, signal an error if D is a list that is not a proper set, i.e., that is not sorted, or has holes, or contains duplicates.

    gap> Size( GaussianIntegers );
    "infinity"
    gap> Size( D12 );
    12 

The default function to compute the size of a domain is DomainOps.Size, which computes the set of elements of the domain with the function Elements (see Elements) and returns the length of this set. This default function is overlaid in practically every domain.