  The intermediate period gap, discovered by Kepler, is an observed dearth of
stellar rotation periods in the temperature-period diagram at $\sim$ 20 days
for G dwarfs and up to $\sim$ 30 days for early-M dwarfs. However, because
Kepler mainly targeted solar-like stars, there is a lack of measured periods
for M dwarfs, especially those at the fully convective limit. Therefore it is
unclear if the intermediate period gap exists for mid- to late-M dwarfs. Here,
we present a period catalog containing 40,553 rotation periods (9,535 periods
$>$ 10 days), measured using the Zwicky Transient Facility (ZTF). To measure
these periods, we developed a simple pipeline that improves directly on the ZTF
archival light curves and reduces the photometric scatter by 26%, on average.
This new catalog spans a range of stellar temperatures that connect samples
from Kepler with MEarth, a ground-based time domain survey of bright M-dwarfs,
and reveals that the intermediate period gap closes at the theoretically
predicted location of the fully convective boundary ($G_{\rm BP} - G_{\rm RP}
\sim 2.45$ mag). This result supports the hypothesis that the gap is caused by
core-envelope interactions. Using gyro-kinematic ages, we also find a potential
rapid spin-down of stars across this period gap.
