Is it possible to measure constant velocity using accelerometer/Gyro
Integration of an acceleration is not a velocity alone, but velocity due to acceleration over time, plus the constant initial velocity. 

Therefore the rationale of integrating acceleration to determine velocity is invalid, unless the initial velocity is provable to be zero.

The answer: No, an accelerometer, or a gyroscope, can not be used to determine constant velocity, without some additional sensor, or baseline data i.e. initial velocity.