timba – absolute pressure is relative to vacuum, gauge pressure is relative to atmospheric.
It’s possible – though unlikely in the case of a bike pump – that the gauge is calibrated in absolute pressure. In this case, the gauge would read 14.7psi/1bar at rest (at sea level). I’m defining at rest to mean not hooked up to a tyre/thumb over valve and pumping/whatever. This should really be clearly identified by appending an “a” to the units (psia, bara).
Assuming the gauge reads “0” at rest then it’s calibrated in gauge pressure. Strictly speaking, this should have “g” added to the end of the units (psig, barg). But because it’s what we’re on about with tyre pressures, this is usually missed off pressure gauges on pumps car/bike/shock pumps. Gauge pressure is effectively the differential pressure between atmospheric pressure and the pressure within the pressurised body (e.g. tyre, shock, fork) that you’re interested in.
If you thing about it… a tyre at a pressure of anything less than atmospheric pressure (14.7psia at sea level) would not resist the atmospheric pressure bearing down on it and would be crushed. As a result, it wouldn’t be at all effective as a tyre.