WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
If you look at the datasheet for a KT88 you will see the average transconductance for a specific operating point is 11.5mA/V, or 115 micromhos. For the mutual conductance tester you have they might have a different operating point but still 120 is close to the 115 in the datasheet. You will want a new tube to test around this 120 mark in your tester.
So in short you want the meter to read 120 for a new tube.
If that were how this tester worked, then a lower gm tube would test as "bad". Since I have a 707, I can say that it is more of a percentage scale, allowing 20% overage for a particularly strong tube. The sensitivity is set according to the type of tube being tested. So, to answer the question, 100 or better would be ideal. In practice, with mostly used tubes, I have found very few test above 100, but many around 80-90.