  Motivated by the NANOGrav 15 year data and other recent investigations of
stochastic gravitational background radiation based on pulsar timing arrays, we
show how superheavy strings survive inflation but the slightly heavier
monopoles do not in a non-supersymmetric hybrid inflation model based on
flipped $SU(5)$. With the dimensionless string tension parameter $G \mu\sim
10^{-6}$, the gravitational wave spectrum emitted by the strings, which are
metastable due to breaking caused by monopole-antimonopole quantum mechanical
tunneling, is compatible with the latest NANOGrav measurement as well as the
advanced LIGO-VIRGO third run data. The string network undergoes about 30
$e$-foldings of inflation which suppresses the spectrum in the LIGO-VIRGO
frequency range. With the symmetry breaking chain $SU(5) \times U(1)_X \to
SU(3)_c \times SU(2)_L\times U(1)_Z \times U(1)_X \to SU(3)_c \times SU(2)_L
\times U(1)_ Y$, the estimated proton lifetime is of order $10^{36}-10^{37}$
yrs.
