  In this work, we study microlensing effects in strongly lensed gravitational
wave (GW) signals corresponding to global minima in galaxy-scale lenses. We
find that stellar microlenses alone are unable to introduce noticeable wave
effects in the global minima GW signals at strong lensing magnification
$(\mu)<50$ with match value between unlensed and lensed GW signals being above
${\sim}99.5\%$ in ${\sim}90\%$ of systems. Since the stellar microlenses
introduce negligible wave effects in global minima, they can be used to probe
the intermediate-mass black hole (IMBH) lenses in the galaxy lens. We show that
the presence of an IMBH lens with mass in the range $[50,10^3]~{\rm M_\odot}$
such that the global minima lies within five Einstein radius of it, the
microlensing effects at $f<10^2$ Hz are mainly determined by the IMBH lens for
$\mu<50$. Assuming that a typical strong lensing magnification of 3.8 and high
enough signal-to-noise ratio (in the range $\sim[10, 30]$) to detect the
microlensing effect in GW signals corresponding to global minima, with
non-detection of microlensing effects in ${\sim}15 ({\sim}150)$ lensed GW
signals, we can rule out dark matter fraction $>10\% (>1\%)$ made of IMBH
population inside galaxy lenses in mass range $[50, 10^3] {\rm M_\odot}$ with
${\sim}$90\% confidence.

Cosmology and Nongalactic Astrophysics