  We report the results of a pilot program for a Green Bank Telescope (GBT)
MUSTANG Galactic Plane survey at 3 mm (90 GHz), MGPS90. The survey achieves a
typical $1\sigma$ depth of $1-2$ mJy beam$^{-1}$ with a 9" beam. We describe
the survey parameters, quality assessment process, cataloging, and comparison
with other data sets. We have identified 709 sources over seven observed fields
selecting some of the most prominent millimeter-bright regions between $0\deg <
\ell < 50\deg$ (total area $\approx 7.5 \deg^2$). The majority of these sources
have counterparts at other wavelengths. By applying flux selection criteria to
these sources, we successfully recovered several known hypercompact HII (HCHII)
regions, but did not confirm any new ones. We identify 126 sources that have
mm-wavelength counterparts but do not have cm-wavelength counterparts and are
therefore candidate HCHII regions; of these, 10 are morphologically compact and
are strong candidates for new HCHII regions. Given the limited number of
candidates in the extended area in this survey compared to the relatively large
numbers seen in protoclusters W51 and W49, it appears that most HCHII regions
exist within dense protoclusters. Comparing the counts of HCHII to ultracompact
HII (UCHII) regions, we infer the HCHII region lifetime is 16-46% that of the
UCHII region lifetime. We additionally separated the 3 mm emission into dust
and free-free emission by comparing with archival 870 $\mu$m and 20 cm data. In
the selected pilot fields, most ($\gtrsim80$%) of the 3 mm emission comes from
plasma, either through free-free or synchrotron emission.
