NavList:

The  Admiralty Research Laboratory at Teddington examined one and compared it with a Hughes bubble sextant and found it lacking. I have tried to upload my pdf file of the report to the NavList site but receive an upload error. I can't remember how I obtained the original file, but googling "gyro sextant admiralty" brings up one way.

Here are the conclusions: 
6, Conclusions.
The gyro sextant in the form described is not considered
satisfactory. Extreme care in handling is necessary and since the gyro
line of sight is maintained in the horizontal by the resultant direction
of gravity it is improbable that accurate results will be obtained on a
rolling ship. The bubble sextant is more convenient and gives
results of equal or better accuracy. Either type of Sextant should be
capable of giving a fix at sea, under calm conditions, within one to
three miles, and provided a large number of observations are made
similar accuracy may be obtainable in a moderate sea

Bill Morris

Pukenui

New Zealand