Different valve size and rating , I need to order Gear Box for I have experienced alot that Gear Box get damage .
How can I specify  and order different Gear Box to keep them stand by ?
Normally the valve manufacturer must make the interface from the gear box to the  gate valve.  Therefore I suggest you start your search with the valve manufacturer. 
There are many manufacturers spacialize on actuation of all types of valves, who can fulfill your needs. ( BRAY, AUMA, ROTORK etc.).I'm sure that you find something for you there.    Take attention to flange connection between valve and gear box, the nominal torque of the gear box and the necessary torque for actuating the valve.

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