what is so special about Gm measurement?
The U61 has Gm-values for each tube in his measurement booklet. You then need to vary the emission-setting of G1 plus-and-minus 0,5V and get Gm by evaluating the corresponding excursion of the anode current meter. Of course you cannot read it directly on a dial, and if you want to have Gm for other settings it gets tiresome.
Or is Gm something else than delta-anode-current / delta-grid-voltage?