  Neutral hydrogen (HI) is the fundamental component of the interstellar
medium. The Galactic Plane Pulsar Snapshot (GPPS) survey is designed for
hunting pulsars by using the Five-hundred-meter Aperture Spherical radio
Telescope (FAST) from the visible Galactic plane within $|b| \leq 10^{\circ}$.
The survey observations are conducted with the L-band 19-beam receiver in the
frequency range of 1.0 $-$ 1.5 GHz, and each pointing has an integration time
of 5 minutes. The piggyback spectral data simultaneously recorded during the
FAST GPPS survey are great resources for studies on the Galactic HI
distribution and ionized gas. We process the piggyback HI data of the FAST GPPS
survey in the region of $33^{\circ} \leq l \leq 55^{\circ}$ and $|b| \leq
2^{\circ}$. The rms of the data cube is found to be approximately 40 mK at a
velocity resolution of $0.1$ km s$^{-1}$, placing it the most sensitive
observations of the Galactic HI by far. The high velocity resolution and high
sensitivity of the FAST GPPS HI data enable us to detect weak exquisite HI
structures in the interstellar medium. HI absorption line with great details
can be obtained against bright continuum sources. The FAST GPPS survey
piggyback HI data cube will be released and updated on the web:
http://zmtt.bao.ac.cn/MilkyWayFAST/.
