Vielleicht brauchst du sowas?:

Code:
With x as (Select GNR, TNR, ZNR, Max(VGZT) MaxVGZT
             From YourFile
             Group By GNR, TNR, ZNR),
 y as (select * from YourFile)
Select y.seq, y.PNR, Sum(x.MaxVGZT)
From X, Y
where x.gnr = y.gnr
And   x.tnr = y.tnr
and   x.znr = y.znr;
(ungetestet)