Using gyp board/sheetrock goes with also using a much lower response modification factor in your calculations. It depends on the situation, but it's often more beneficial to use OSB/plywood to get the shear value needed with R=6.5 and then install the sheetrock over the plywood/OSB. And using the plywood shear value on the outside and adding the sheetrock shear value on the inside is never going to be worth it because even if you add, for example, 240#/ft for the plywood and 60#/ft for the sheetrock = 300#/ft total (a 25% increase), the base shear will go up by a factor of 6.5/2 based on the lowering of the response modification factor to account for use of sheetrock (a 325% increase).
In the UBC, there used to be, and I believe still is in the IBC or ASCE7, a code restriction of using dissimilar materials, and adding their values on the same wall line. You cannot mix plywood or OSB with GWB or GSB. Moreover ASCE7 Section 12.2.3 applies so that the GSB/GWB values for design would govern.
I don't know which code section prohibits gypboard from being use a shear wall. I have used gypboard as shear wall. it was for a small freestanding office space inside a large warehouse so the gypboard walls over metal stud is my lateral system. Like hikeandgolf mentioned above, the R is much lower than plywood but I had plenty of wall length so that wasn't a problem.
