Yes, Spektrum DSP75 servos will work with Futaba gy401 gyros. The note on the tower hobbies page is in reference to the 8700G servo which is a PCM servo designed to only work with the G7000T gyro. The newer 8900G should work just fine with a gy401 because it uses the standard signaling, but the 8900G has a deadband of zero compared to most digital servos with a deadband of around 3 micro-seconds.

The Spektrum DSP75 can handle the high frame rate from the gyro, the E-flight DS75 can't.

Good luck!