  In the Next-to-Minimal Supersymmetric Standard Model (NMSSM), all
singlet-dominated particles including one neutralino, one CP-odd Higgs boson
and one CP-even Higgs boson can be simultaneously lighter than about 100 GeV.
Consequently, dark matter (DM) in the NMSSM can annihilate into multiple final
states to explain the galactic center gamma-ray excess (GCE). In this work we
take into account the foreground and background uncertainties for the GCE and
investigate these explanations. We carry out a sophisticated scan over the
NMSSM parameter space by considering various experimental constraints such as
the Higgs data, $B$-physics observables, DM relic desnity, LUX experiment and
the dSphs constraints. Then for each surviving parameter point we perform a fit
to the GCE spectrum by using the correlation matrix that incorporates both the
statistical and systematic uncertainties of the measured excess. After
examining the properties of the obtained GCE solutions, we conclude that the
GCE can be well explained by the pure annihilations $\tilde{\chi}_1^0
\tilde{\chi}_1^0 \to b \bar{b} $ and $\tilde{\chi}_1^0 \tilde{\chi}_1^0 \to A_1
H_i $ with $A_1$ being the lighter singlet-dominated CP-odd Higgs boson and
$H_i$ denoting the singlet-dominated CP-even Higgs boson or SM-like Higgs
boson, and it can also be explained by the mixed annihilation $\tilde{\chi}_1^0
\tilde{\chi}_1^0 \to W^+ W^-, A_1 H_1$. Among these annihilation channels,
$\tilde{\chi}_1^0 \tilde{\chi}_1^0 \to A_1 H_i $ can provide the best
interpretation with the corresponding $p$-value reaching 0.55. We also discuss
to what extent the future DM direct detection experiments can explore the GCE
solutions and conclude that the XENON-1T experiment is very promising in
testing nearly all the solutions.
