  In this paper, we investigated four applications of the gauge transformation
for the BKP hierarchy. Firstly, it is found that the orbit of the gauge
transformation for the constrained BKP hierarchy defines a special $(2
+1)$-dimensional Toda lattice equation structure. Then the tau function of the
BKP hierarchy generated by the gauge transformation is showed to be the
Pfaffian. And the higher Fay-like identities for the BKP hierarchy is also
obtained through the gauge transformation. At last, the compatibility between
the additional symmetry and the gauge transformation of the BKP hierarchy is
proved.
