I don't think anyone will dispute that Harbor Freight (
or Hazard Fraught as coined by AvE) makes low quality items but for something that you are only going to use a few times like an injection pressure gauge it is hard to ignore. It gives consistent back-to-back readings which is all you need.
Also regarding pressures in the system.
You should see 29 PSI (
2 bar) at the injector rail when running. You should never see over 40 PSI under any conditions such as; pump on but not running; vacuum control lines removed; etc.