(Matlab) How do I know what's the best fit method to a set of data?

Hi all, I have quite a few questions here hoping to get help with please.

1) I know about Gaussian fit, but what if the curve is up-side-down of a Gaussian (like the one in the graph below), what are they called? 'Inverse Gaussian'?

2) I can do Gaussian fit to data with 1 peak only (and the peak would be in the y+ direction). but I dont know how to fit 'inverse gaussian' data -- which is when the peak is pointing in the y- direction.

3) when there is more than 1 peak (or in this case, more than 1 inverse peak), how do i choose what equation/function I should fit the curve with? in this uploaded graph, i used the 'cftool' and chose 'Gaussian-6' fit, it doesn't do exactly what i expect, but i dont know where is the correct method to start with. please help! when i get a set of data, how to know or do the best fit of it?

Many thanks!