Gamma conjecture I for flag varieties

We prove Gamma conjecture I for all flag varieties by following a strategy proposed by Galkin and Iritani. The main new ingredient is showing that the totally positive part of the Rietsch mirror is mirror to the $\widehat{\mathrm{\Gamma }}$-class and contains the critical point of the superpotential that corresponds to the Perron-Frobenius eigenvalue on the A-side.