PositionClass( G, c )
G must be a domain for which ConjugacyClasses is defined and c must
be an element of G. This functions returns  a positive integer i such
that c in ConjugacyClasses( G )[i].
    gap> G := Group( (1,2)(3,4), (1,2,3,4,5) );;
    gap> ConjugacyClasses( G );
    [ ConjugacyClass( Group( (1,2)(3,4), (1,2,3,4,5) ), () ), 
      ConjugacyClass( Group( (1,2)(3,4), (1,2,3,4,5) ), (3,4,5) ), 
      ConjugacyClass( Group( (1,2)(3,4), (1,2,3,4,5) ), (2,3)(4,5) ), 
      ConjugacyClass( Group( (1,2)(3,4), (1,2,3,4,5) ), (1,2,3,4,5) ), 
      ConjugacyClass( Group( (1,2)(3,4), (1,2,3,4,5) ), (1,2,3,5,4) ) ]
    gap>  g := Random( G );
    (1,2,5,4,3)
    gap> PositionClass( G, g );
    5 
GAP 3.4.4