  This work studies microlensing effects in strongly lensed gravitational wave
(GW) signals corresponding to global minima images 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 implying that GW signals
corresponding to global minima can be treated as reference signal to determine
the amount of microlensing in other strongly lensed counterparts. 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
${\simeq}[10, 30]$) to detect the microlensing effect in GW signals
corresponding to global minima, with non-detection of IMBH-led microlensing
effects in ${\simeq}15\:({\simeq}150)$ lensed GW signals, we can rule out dark
matter fraction $>10\%\:(>1\%)$ made of IMBH population inside galaxy lenses
with mass values $>150 {\rm M_\odot}$ with ${\sim}90\%$ confidence. Although we
have specifically used IMBHs as an example, the same analysis applies to any
subhalo (or compact objects) with lensing masses (i.e., the total mass inside
Einstein radius) satisfying the above criterion.
