Figured it out with some help from the good folks at rcgroups.com.
The WK2618V-D is a 7 channel receiver, with gyro control on channel 7. I had configured things to be a 6 channel receiver, so it bound but the gyro just stayed in rate mode.
Another symptom of "not enough channels" is that the "bound" light on the Rx doesn't go on, even though the Rx is bound. Odd.
Do not ask me questions via PM. Ask in the forums, where I'll answer if I can.
My remotely piloted vehicle ("drone") is a yacht.