How can I make 9 patch of this image when only the center stretched? (I'm working with libgdx) I make the 9patch using google tool
I do not think you can do that in this specific case. As I know the strech patchs has to cross the whole image (by definition), so you can't keep the aspect of this border using 9patch.
Plus, you are using gradien colors for the center of your button, you won't be able to strech that in the 9patch way.
You'd better to have a high resolution image of your button that you'll strech in libGDX taking care of width/heigh ratio.