do we really need a tension gauge ...........

big lad said:

.........for wheel building??

It's all about balance, you've got skill, effort, tools on one side, and results on the other. A shortage on one point can to some extent be compensated by another.

An adequate wheel can be built by someone with moderate skills, a few tools and a sizeable effort. If you've got fewer skills you have to have better tools and/or spend more time at it to get a similar result. To build the ultimate wheel you need it all; time, tools and skill.

I've done so much with so little for so long I could probably do almost anything with nothing at all. Just play it like a piano and look for the sour notes.

I agree with Peet. I have just built my first wheel (rear) with none of the specialist tools and it has worked out fine. I was tempted by all the glittering tensiometers, dish sticks, truing stands etc. then thought "what have I got to lose?" (Actually, 36 spokes and a rather nice rim, but that's irrelevant because I succeeded.)