  We assess proton decay signatures in the simplest viable $SU(5)$ model with
regard to constraints on parameters governing the Standard Model fermion mass
spectrum. Experimental signals for all eight two-body proton decay processes
result from exchange of two gauge bosons, a single scalar leptoquark, or their
combination. Consequently, it enables us to delve into an in-depth anatomy of
proton decay modes and anticipate future signatures. Our findings dictate that
observing a proton decay into $p\to\pi^0e^+$ indicates gauge boson mediation,
with the potential for observation of $p\to\eta^0e^+$ mode. Alternatively, if
decay is through $p\to K^+\overline\nu$ process, it is mediated by a scalar
leptoquark, possibly allowing the observation of $p\to\pi^0\mu^+$. Detection of
both $p\to\pi^0 e^+$ and $p\to K^+\overline\nu$ could enhance $p\to\pi^0\mu^+$
through constructive interference. The model predicts inaccessibility of
$p\to\pi^+\overline\nu$, $p\to\eta^0\mu^+$, $p\to K^0e^+$, and $p\to K^0\mu^+$,
regardless of the dominant mediation type, in the coming decades. In summary,
through a comprehensive analysis of proton decay signals, gauge coupling
unification, and fermion masses and mixing, we precisely constrain the
parameter space of the $SU(5)$ model in question.
