  We use HI and H2 global gas measurements of galaxies from xGASS and xCOLD
GASS to investigate quenching paths of galaxies below the star formation main
sequence (SFMS). We show that the population of galaxies below the SFMS is not
a 1:1 match with the population of galaxies below the HI and H2 gas fraction
scaling relations. Some galaxies in the transition zone (TZ) 1-sigma below the
SFMS can be as HI-rich as those in the SFMS, and have on average longer gas
depletion timescales. We find evidence for environmental quenching of
satellites, but central galaxies in the TZ defy simple quenching pathways. Some
of these so-called "quenched" galaxies may still have significant gas
reservoirs and be unlikely to deplete them anytime soon. As such, a correct
model of galaxy quenching cannot be inferred with SFR (or other optical
observables) alone, but must include observations of the cold gas. We also find
that internal structure (particularly, the spatial distribution of old and
young stellar populations) plays a significant role in regulating the star
formation of gas-rich isolated TZ galaxies, suggesting the importance of bulges
in their evolution.
