Changing submit button style

If you're using the filter, then that overrides the form settings. You would need to change the button text directly in the filter/markup.

add_filter("gform_submit_button", "form_submit_button", 10, 2);
function form_submit_button($button, $form){
return "<button class='btn btn-warning btn-large' id='gform_submit_button_{$form["id"]}'><div class='center'>YOUR BUTTON TEXT HERE</div></button>";
}

Do you need an actual "button" element for some reason? If not, you can leave that out all together and use the default "submit" type input and the verbiage can be changed directly from the form settings.

Also, sure, centering the button or submit element is just a matter of applying the proper CSS rules. If you would like to post a URL to your some, someone here can take a look and help you with a proper CSS snippet to get it positioned correctly.

Just looked at your form and it appears you have the button styled and centered?

screenshot: http://bit.ly/WymPuo

Did you still need some help or have you sorted it all out?

Well, you can use the filter mentioned above to completely re-write the button markup and assign whatever classes you want.

http://www.gravityhelp.com/documentation/page/Gform_submit_button

Other than that, you can use more specific CSS inheritance to override any existing class definitions and to add new styling.

You can apply a unique class name to the form itself in the form settings. From there, you can use simple CSS inheritance to target the submit button. Here's a quick example.. I added the class name "someform" in the main form settings

[css]
body .gform_wrapper.someform_wrapper .gform_footer input.gform_button {
	font-size:1.6em;
	color: blue
}

screenshot: http://bit.ly/Sp7tWK

Also, if you inspect the markup, you'll see that the submit button has a unique ID besides the class that you can target for CSS manipulation. Either way should work just fine for you.