I always thought it somewhat of a scam for the certification agencies to not teach and demand training in the use of an ip gauge. People spend over $1000 for a computer that has ( in my opinion) marginal incremental value over a cheap one, for a rec diver. yet people aren’t made aware of the utility and potential increase in safety that could be derived from an additional $20 for the device. Perhaps it is because the shops fear a reduction on service fees? The marginal utility of having the ability to check regulators before a big trip is huge.
What's to train?
Attach the IP gauge to your inflator hose and open a valve; but, as already mentioned on another thread, just the use of an IP gauge is all-but useless unless you can both recognize the significance of a reading and / or are capable of safely "tuning" or "detuning" regulators, as the case might be, should those numbers be off.
What otherwise would be the point?
That's way beyond the purview of any OW course, off in the land of @rsingler -- and well beyond what any scuba rental fleet would ever allow, in terms of futzing with their gear . . .