Button Fixing

The way your theme handles the button and the way Gravity Forms handles the button are completely different. The theme is styling a span, where in Gravity Forms they are inputs. The markup on the buttons is not the same which means the CSS in your theme won't be applied to the Gravity Forms submit button by default.

That said, you can try to copy the CSS for the buttons in your theme and then use those styles to target the Gravity Forms submit button. I don't think it will go well though, since the theme uses a span and Gravity Form uses an input form element.

An image is quickest, but would not respond to text changes, if you needed that. The CSS they have works well when creating buttons out of text of variable length. If you don't need that for this particular button, you could add a button to your site somewhere using the text you want on the button (Submit?) and then take a screen capture and edit that button down to the right size. You would have to create a hover state for the button too. But I think that is going to be quicker than trying to copy their approach to styling spans and make it apply to Gravity Forms inputs.