  In this paper, we analytically study the phase structure and construct the
Ruppeiner geometry in the extended phase space for the five-dimensional neutral
Gauss-Bonnet AdS black hole. Through calculating the scalar curvature of the
Ruppeiner geometry and combining the phase transition, we show that the
attractive interaction is dominant in the microstructure of the black hole
system. More significantly, there is an intriguing property that the normalized
scalar curvature has the same expression for the saturated small and large
black hole curves. This implies that although the microstructure is different
before and after the small-large black hole phase transition, the interaction
between the microscopic constituents keeps unchanged. These results are quite
valuable on further understanding the microstructure of the AdS black hole in
modified gravity.
