The gate valve spec. is Class 300 lbs SS body BW Sch40S Man Operated.  The design conditions are T=200F and P=180 psig, in water.  The Cv required is 3100.  I came across with a similar spec that uses a trim characteristic of "Double Seat".  My knowledge of valves is limited.  Can someone help to understand what is "Double seat"? 
In addition, can this trim characteristic achieve the Cv in a manual gate valve?
Almost all gate valves are double seat.  The gate drops in between the seats and is wedged into place by various mechanisms.  Solid wedge, flex wedge, split wedge, and double disc parallel seat (with an internal wedge).
Knife-gate valves tend to be single-seat but certainly not used in Nuclear applications-usually slurry such as pulp.  
Frequently Cv for gate valves is not even published because they are NOT for throttling. However, it would be highly unlikely for a 10" Gate valve to have a Cv that is not greater than 3100.  As Asher said:essentially straight pipe.
Gate valves have metal-to-metal seats and it takes a huge specific loading to get them to shut off tightly. Basically it is necessary to re-coin the seat each time.  Hence the wedge. Other handy associated devices for gate valves are the hammer-blow handwheel, and the large maintenance ape with the long cheater bar. 

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