Transconductance is a derived result calculated from several measurements. It is a measurement of how much AC current will pass through a tube at a given plate voltage, bias voltage, and signal input. Mathematically, it’s the result of the AC output current divided by the AC input voltage. Different results will occur given different driving environments for a tube. You can see this on a Maxi-Matcher by changing plate voltage or bias voltage and watching the transconductance numbers go up or down as a result.
When driving a tube, reducing the grid voltage generates more current flow, which makes transconductance results go up. Increasing the plate voltage also makes results go up. What makes it meaningful is to set up a similar test environment for a given tube type and then compare all the tubes of that type in that set up, to each other. This is what all tube testers do, from vintage Hickoks to modern Maxi-Matchers. Each different tester has its own set of values it applies to a tube, and generates its own set of measurements.
The specifications for transconductance that are shown on manufacturer’s product sheets are always maximum values; the maximum AC current a tube can pass. In fact, the specifications published by the manufacturers were used to sell tubes, not as usage values. It’s similar to maximum engine RPM or redline for a car – you don’t actually use the engine that way, but it can go that fast. Tubes are seldom used at these max values, because they would burn out in a short time.