Your smog technician is correct. Unfortunately, if the vehicle's emissions system was not designed to incorporate a Boost Pressure gauge, installing one, especially tapping vacuum with a T connector, will cause the vehicle to fail the smog test under the category of "Tamper".
The only time such gauges are allowed are when they are installed by the factory at the time of manufacturing; and upon manufacturing, received California Air Resources Board approval.
The confusion arises when the same make and model vehicle you own, also comes from the factory with a performance option which you want to install later. Is it legal for you to do this? This question can only be answered on a case by case basis and depending on the intrusiveness of the modification, i.e. does it require the computer to be re-calibrated, will you be tapping into vacuum lines which are not meant to be tapped into, and the like.
Your best option is to contact the vehicle manufacturer and ask whether or not your engine's classification is permitted to have equipped the modification component you wish to add. If the exact same engine comes in more than one emission configuration and does not need computer programming (or the computer can be programmed to accept the new component), you will be able to legally pass the inspection, but must do so (one-time only) at a State run smog check referee station (call 800-622-7733 for an appointment). Subsequent inspections can be completed at privately owned smog shops.
