Generalized Additive Models with R gam

I have fitted for the first time a GAM with smoothing splines. Since I’m new to GAMs, what features of the output ‘summary(gam)’ I should look at, besides the sig. of the param. and nonparam. effects? Are there any useful diagnostics, as in standard Multiple Regression? Finally, how can I estimate the value of FM(‘x’) at which the slope of the nonlinear, quadratic fit changes direction (see attached figure)? Thanks!

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