  (Abridged) Observational estimates of galaxy properties rely on the inherent
galaxy-wide initial mass function (gwIMF), which systematically varies with the
global SFR and metallicity, as proposed by the integrated-galactic IMF (IGIMF)
theory and supported by empirical evidence. We incorporate PARSEC and COLIBRI
stellar isochrones into the GalIMF code, a galaxy chemical evolution (GCE)
model featuring real-time updates of environment-dependent gwIMFs. This newly
developed photometric GalIMF (photGalIMF) code allows the calculation of
photometric properties for galaxies with diverse stellar populations.
Subsequently, we analyze observed luminosities and metallicities of local
star-forming galaxies to deduce their stellar masses assuming that they have
constant SFRs over 13.6 Gyr. We also compute SFR$-$H$\alpha$ luminosity
relations for varying stellar metallicities using a separate stellar population
synthesis code based on PEGASE. Comparing the IGIMF theory to the canonical
universal IMF, our analysis reveals that estimates of the stellar masses and
SFRs for local star-forming galaxies differ by factors of $\approx 2$ and 10,
respectively. The computed gas-depletion timescale increases with gas mass,
implying lower star formation efficiencies in more massive galaxies, possibly
due to stronger feedback regulation, aligning with theoretical expectations.
Additionally, the characteristic stellar mass buildup timescale increases with
stellar mass, indicating that massive disk galaxies initiate star formation
earlier than their low-mass counterparts. The photGalIMF code enables
self-consistent computations of galactic photometry, self-consistently with GCE
modelling within the context of an environment-dependent gwIMF. Utilizing
Ks-band and H$\alpha$ luminosities of galaxies, the outcomes include galaxy
mass, SFR, and fitting functions for the SFR correction factor.
