convert the next, previous and/or submit button <input> to <button> and uses the input value to create the button text inside a <span>, this means you can set the button text by editing the form or page break settings.

button_filters.php

<?php
/**
 * Filters the next, previous and submit buttons.
 * Replaces the forms <input> buttons with <button> while maintaining attributes from original <input>.
 * @param string $button Contains the <input> tag to be filtered.
 * @param object $form Contains all the properties of the current form.
 * @return string The filtered button.
 */
add_filter( 'gform_next_button', 'input_to_button', 10, 2 );
add_filter( 'gform_previous_button', 'input_to_button', 10, 2 );
add_filter( 'gform_submit_button', 'input_to_button', 10, 2 );
function input_to_button( $button, $form ) {
    $dom = new DOMDocument();
    $dom->loadHTML( $button );
    $input = $dom->getElementsByTagName( 'input' )->item(0);
    $new_button = $dom->createElement( 'button' );
    $button_span = $dom->createElement( 'span', $input->getAttribute( 'value' ) );
    $new_button->appendChild( $button_span );
    $input->removeAttribute( 'value' );
    foreach( $input->attributes as $attribute ) {
        $new_button->setAttribute( $attribute->name, $attribute->value );
    }
    $input->parentNode->replaceChild( $new_button, $input );
    return $dom->saveHtml( $new_button );
}

Hi, I just found your awesome idea to replace input buttons by

copy/pasted code into functions.php ... no errors ... but I have just no buttons anymore now.
But I suppose I need to execute the function 'input_to_button' to get this working, but how?

Thx a lot!

If the add_filter lines are there the function will run. What version of PHP are you running, the ability to include a parameter in the saveHTML call was only introduced in PHP 5.3.6.

works like a charm after PHP 5.3.3 update to 5.4
thx a lot!

Using DOMDocument and everything. You sir, have done very well here. I don't have to adjust a single thing :)

Maybe one thing: why add the span granularity? Why not have input "value" attribute simply as button text node?

$new_button->appendChild( $dom->createTextNode( $input->getAttribute( "value" ) ) );