Gate valves are very loosely guided.  You may be able to calculate a Cv for an intermediate position, but in reality the disc will be dancing about and the Cv will be dithering back and forth, without commanded movement of the disc. 
If, in fact, you intend to throttle continuously with a gate valve, you will find that any guiding surfaces provided will be fretted away and the disc will be found mysteriously, in the middle of the night, downstream, lodged in something that will be expensive to repair. 
I will respectfully submit that the reason that the Cv curve is not published for a Gate Valve is because Gate Valve manufacturers know not to use the gate valve for modulating, and don't want to give anybody any information that suggests such a use.  Gate valves are not intended for throttling service.  However, I have found information on the resistance of partially open gate valves.  The information was published in Section 9 of the "Handbook of Hydraulic Resistance - Coefficients of Local Resistance and of Friction" by I. E. Idel'chik in 1966.  This documnent was distrbuted the National Technical Information Service of the U.S. Department of Commerce.

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