How to edit the font of the subtitle in the gogetslider?

You should go to menu Theme Options -> Typography -> Body Typography.

Do not forget to save changes.

But this changes not only slider subtitle typography. If you want to change only it, you can use this custom css code into style.css file via menu Appearance -> Editor.

.header-content.skrollable.skrollable-between {
font-family: impact ;
}

But do not forget to set own font-family.