Unfortunately, to do it the way I’m proposing requires SASS for now. I’m thinking about publishing a compiled version of that CSS here as well.
“Did you guys tried to use the Boostrap Style on inputs like this example with URL Address?”
— The problem with this is Gravity Forms doesn’t let you edit the styles of the inputs themselves, just the style of the wrapper element. So there’s no way to apply the default Bootstrap styles, which means you have to take Bootstrap CSS and apply it to default Gravity form classes.