A researcher in Russia performed experiments which saw odd southerly deflections which he believed to defy classical mechanics. The gyrocompass is probably taking advantage of this effect he found.
http://www.chronos.msu.ru/old/EREPORTS/levich_substan_inter/levich_substan_inter.htmOne of Kozyrev’s early papers (Kozyrev 1958) describes in detail the experiments on anomalous southern deflection of bodies falling in the Earth’s gravitational field, from Hooke’s experiments of 1680 to that of Kozyrev, arranged at Pulkovo.
The northern deflection effect “is observed by vibration of any nonrotating body suspended to a filament. The conditions necessary for obtaining the pendulum displacement effect due to the Earth’s rotation, are quite similar to the above conditions of gyro displacement... These experiments were carried out at Pulkovo with pendulums of different lengths, from 1.5 to 11 meters. The fractional displacement turned out to be independent of pendulum length... Thus the displacement is caused by a certain force, independent of pendulum construction. This force significantly varies with geographic latitude... No doubt, the forces measured by pendulum displacements are horizontal projections of asymmetric forces active at the surface of the Earth. As for vertical projections of the same asymmetric forces, one can try to obtain them by vibrating the system with a vertical degree of freedom” (Kozyrev 1963, p.107). “The simplest system of this kind is a beam balance. Assume that a weight is suspended at one end of the beam by a rigid suspension able to transfer beam vibrations to the weight. The other, balancing weight was suspended to rubber shock-absorbers which damp all the oscillations. Then, as the balance vibrates, it is possible to observe oscillating weight reduction...
He believed that the earth was round, and speculated on some kind of force inherent in nature that deflected southwards, but it is difficult to understand his exact theory. He did perform several experiments which showed southern deflection, whatever his theory about it might be. There are a bunch of books and articles about him if of interest.
Oddly, the deflections and and changes of weight that he found only seem to occur with moving mechanical objects, such as gyroscopes.
Assessing the operation of the gyrocompass shows that it indeed operates on slight weight changes.
See:
https ://books.google.com/books?id=PiEDAAAAMBAJ&lpg=PA91&ots=s0slhdUupv&pg=PA91#v=onepage&q&f=false