  In gravitational wave (GW) astronomy accurate measurement of the source
parameters, such as mass, relies on accurate waveform templates. Currently, the
templates are developed assuming that the source, such as a binary black hole
(BBH), is residing in a vacuum. However, astrophysical models predict that BBHs
could form in gaseous environments, such as common envelops, stellar cores, and
accretion disks of active galactic nuclei. Here we revisit the impact of gas on
the GW waveforms of stellar-mass BBHs with a focus on the early inspiral phase
when the GW frequency is around milli-Hertz. We show that for these BBHs, gas
friction could dominate the dynamical evolution and hence duplicate chirp
signals. The relevant hydrodynamical timescale, $\tau_{\rm gas}$, could be much
shorter than the GW radiation timescale, $\tau_{\rm gw}$, in the above
astrophysical scenarios. As a result, the observable chirp mass is higher than
the real one by a factor of $(1+\tau_{\rm gw}/\tau_{\rm gas})^{3/5}$ if the gas
effect is ignored in the data analysis. Such an error also results in an
overestimation of the source distance by a factor of $(1+\tau_{\rm
gw}/\tau_{\rm gas})$. By performing matched-filtering analysis in the
milli-Hertz band, we prove that the gas-dominated signals are practically
indistinguishable from the chirp signals of those more massive BBHs residing in
a vacuum environment. Such fake massive objects in the milli-Hertz band, if not
appropriately accounted for in the future, may alter our understanding of the
formation, evolution, and detection of BBHs.
