Relationship between $\langle \nabla f(\overline{x}), x - \overline{x}\rangle > 0$ and the the minimality of $\overline{x}$. Say $f$ is a differentiable function over a convex set $X$ with $\langle \nabla f(\overline{x}), x - \overline{x}\rangle > 0$ for all $x, \overline{x} \in X$ such that $x \neq \overline{x}$. Can we conclude from this alone that $\overline{x}$ is a local minimum? Certainly, if $f$ were pseudoconvex, this would hold trivially. However, I suspect that the strict inequality here may imply that $\overline{x}$ is indeed a local minimum

Your question is closely related to the Frank-Wolfe algorithm to find an optimal solution of a convex program or a "stationary" point in nonconvex problems. The quantity that you defined is known as (negative) surrogate duality gap or in short gap function. A stationary point can be a local minimum, local maximum, or a point in the boundary of the feasible set. You can find a solid discussion on the gap function in [1] and the application of the FW algorithm for nonconvex problems in [2].
[1] Bertsekas, D. P. (1999). Nonlinear programming. Athena Scientific
[2] Lacoste-Julien, S. (2016). Convergence rate of Frank Wolfe for non-convex objectives. arXiv:1607.00345
