One of the major reasons for allocating WAP-5 to GZB is because it is the oldest shed which houses these locos when they were imported from swiss.
Also a lot of NR/NWR routes which were d loco routes are getting electric and there is a heavy demand for e loco links. Since WAP-1,s are almost at the end of their codal life new WAP-5,s are being allocated to GZB.
Nowdays all major IC and express/passenger trains running on rohtak routes are WAP-5,s GZB hauled.