That's true. I suspect that the desire to orient the gauge horizontally is so that the force of gravity will not add or subtract to the hydrostatic force of the air on the plunger itself - in other words, so that the weight of the plunger is not a factor.
To be honest, I think that the weight of the plunger is a completely insignificantly small percentage of the force of the air, and, in addition, friction from wear or dirt is a much larger factor.
The important thing is probably consistency in taking the measurement, and the repeatability of the gauge. I'd rather have a gauge that read 29 PSI EVERY TIME to an actual of 30 PSI, than a gauge that read 29 then 30 then 31 then 30 on four successive readings.