  The accumulation of gluons inside nucleons, i.e., the gluon condensation, may
lead to a characteristic broken power-law gamma-ray spectrum in high-energy
nucleon collisions. Here we show that the observed spectra of at least 25
sources in the second Fermi Large Area Telescope Catalog of Gamma-ray Pulsars
can be well fitted by such a broken power-law function that has only four free
parameters. It strongly indicates that the gamma-ray emission from these
pulsars is of hadronic origin, but with gluon condensation inside hadrons. It
is well known that the quark-gluon distribution in a free nucleon is different
from that in a bound nucleon. This work exposes the nuclear $A$ dependence of
the gluon condensation effect, where $A$ refers to the baryon number. Our study
reveals the gluon condensation under the condition of $A\to\infty$, which may
open a new window for eavesdropping on the structure of compact stars on the
subnuclear level.
