  In this paper, we study the small-large black hole phase transition and
construct the Ruppeiner geometry for the five-dimensional charged
Gauss-Bonnet-AdS black hole in the grand canonical ensemble. By making use of
the equal area law, we obtain the analytical coexistence curve of the small and
large black holes. Then the phase diagrams are examined. We also calculate the
change of the thermodynamic volume during the small-large phase transition,
which indicates that there exists a sudden change among the black hole
microstructures. The corresponding normalized scalar curvature of the Ruppeiner
geometry is also calculated. Combing with the empirical observation of scalar
curvature, we find that for low electric potential, the attractive interaction
dominates among the microstructures, while a high electric potential produces
repulsive interactions. In the reduced parameter space, we observe that only
attractive interaction is allowed when the coexistence region is excluded. The
normalized scalar curvature also admits a critical exponent 2 and a universal
constant $-\frac{1}{8}$. In particular, the value of the normalized scalar
curvature keeps the same along the coexistence small and large black hole
curves. So in the grand canonical ensemble, the interaction can keep constant
at the phase transition where the black hole microstructures change. These
results disclose the intriguing microstructures for the charged AdS black hole
in the Gauss-Bonnet gravity.
