  Giant Low Surface Brightness Galaxies (GLSBGs) are fundamentally distinct
from normal galaxies (LSBGs) in star formation and evolution. In this work, we
collected 27 local GLSBGs. They have high stellar masses (M*>10^10 Msolar) and
low SFRs. With the specific SFRs lower than the characteristic value of the
local star-forming (SF) galaxies of M*=10^10 Msolar(sSFR < 0.1 Gyr^-1), GLSBGs
deviate from the SF main sequence (MS) defined for local SFGs respectively by
E07 and S16 at the high M* regime. They are HI-rich systems with HI gas mass
fractions (fHI) higher than the S16 MS galaxies, but have little molecular gas
(H2), implying a low efficiency of HI-to-H2 transition due to the low HI
surface densities that are far lower than the minimum of 6 - 8 Msolar pc^-2
required for shielding the formed H2 from photodissociation. For GLSBGs, the
inner, bulge-dominated part with lower SFRs and higher M* is the main force
pulling the entire GLSBG off from the MS, while the outer, disk-dominated part
with relatively higher SFRs and lower M* reduces the deviations from the MS.
For some cases, the outer, disk-dominated parts even tend to follow the MS. In
the aspect of NUV - r versus g - r colors, the outer, disk-dominated parts are
blue and behave similarly to the normal star-forming galaxies while the inner,
bulge-dominated parts are in statistics red, indicating an inside-out star
formation mechanism for the GLSBGs. They show few signs of external
interactions in morphology, excluding the recent major merger scenario.
