Hello,
Using this data sheet :-
https://frank.pocnet.net/sheets/126/p/PX4.pdf
The gm of 6mA/V is measured at Va = 100V and Vg = 0V.
Looking at the characteristic curves for Va = 100V:
At Vg = 0V, Ia = 69mA and at Vg = -4V, Ia is about 45mA. which corresponds to a gm of 6mA/V.
However gm does decrease as Vg increases (negatively).
For example at Vg =-8V , Ia= 28mA and at Vg = -12V , Ia = 16mA, which corresponds to a gm of 3mA/V
So the value of gm can depend on the conditions under which it is measured which may well explain the differences between the Avo and Taylor measurements.
Your best bet might be to measure the Va and Vg of your test setup and then use characteristic curves to determine what the gm should be.
Yours, Richard