  Hub-filament systems are suggested to be the birth cradles of high-mass stars
and clusters. We apply the FILFINDER algorithm to the integrated intensity maps
of the 13CO (3-2) line to identify filaments in the G333 complex, and extract
the velocity and intensity along the filament skeleton from moment maps. Clear
velocity and density fluctuations are seen along the filaments, allowing us to
fit velocity gradients around the intensity peaks. The velocity gradients
fitted to the LAsMA data and ALMA data agree with each other over the scales
covered by ALMA observations in the ATOMS survey. Changes of velocity gradient
with scale indicate a ''funnel'' structure of the velocity field in PPV space,
indicative of a smooth, continuously increasing velocity gradient from large to
small scales, and thus consistent with gravitational acceleration. The typical
velocity gradient corresponding to a 1 pc scale is ~1.6km/s/pc. Assuming
free-fall, we estimate a kinematic mass within 1 pc of ~1190 M$_\odot$, which
is consistent with typical masses of clumps in the ATLASGAL survey. We find
direct evidence for gravitational acceleration from comparison of the observed
accelerations to those predicted by free-fall onto dense hubs. On large scales,
we find that the inflow may be driven by the larger scale structure, consistent
with hierarchical structure in the molecular cloud and gas inflow from large to
small scales. The hub-filament structures at different scales may be organized
into a hierarchical system extending up to the largest scales probed, through
the coupling of gravitational centers at different scales. We argue that the
''funnel'' structure in PPV space can be an effective probe for the
gravitational collapse motions in molecular clouds. The large scale gas inflow
is driven by gravity, implying that the molecular clouds in G333 complex may be
in the state of global gravitational collapse.

Astrophysics of Galaxies;Solar and Stellar Astrophysics