Its documented and working for me, are you sure you have the contrib module installed?
>>> help(cv2.structured_light.GrayCodePattern.generate)
Help on method_descriptor:
generate(...)
generate([, patternImages]) -> retval, patternImages
. @brief Generates the structured light pattern to project.
.
. @param patternImages The generated pattern: a vector<Mat>, in which each image is a CV_8U Mat at projector's resolution.