GP4 Tweaker

How to modify damage in GP4 tweaker? I cant find it anywhere

It's the 6th one down, labeled "Damage Coefficient". My personal recommendation is 150%-200%. Anything higher and you might as well be driving paper aeroplanes.

What file is that, is it .exe or .ini? Name of the file please?

It's in the menu of the gp4tweak.exe file. It should look like this;