Helpful function for adding Bootstrap 4 classes to Gravity Forms fields.

Gravity Forms + Bootstrap 4

<?php
/**
 * Gravity Forms Bootstrap Styles
 *
 * Applies bootstrap classes to various common field types.
 * Requires Bootstrap to be in use by the theme.
 *
 * Using this function allows use of Gravity Forms default CSS
 * in conjuction with Bootstrap (benefit for fields types such as Address).
 *
 * @see  gform_field_content
 * @link http://www.gravityhelp.com/documentation/page/Gform_field_content
 *
 * @return string Modified field content
 */
add_filter("gform_field_content", "bootstrap_styles_for_gravityforms_fields", 10, 5);
function bootstrap_styles_for_gravityforms_fields($content, $field, $value, $lead_id, $form_id){

	// Currently only applies to most common field types, but could be expanded.

	if($field["type"] != 'hidden' && $field["type"] != 'list' && $field["type"] != 'multiselect' && $field["type"] != 'checkbox' && $field["type"] != 'fileupload' && $field["type"] != 'date' && $field["type"] != 'html' && $field["type"] != 'address') {
		$content = str_replace('class=\'medium', 'class=\'form-control medium', $content);
	}

	if($field["type"] == 'name' || $field["type"] == 'address') {
		$content = str_replace('<input ', '<input class=\'form-control\' ', $content);
	}

	if($field["type"] == 'textarea') {
		$content = str_replace('class=\'textarea', 'class=\'form-control textarea', $content);
	}

	if($field["type"] == 'checkbox') {
		$content = str_replace('li class=\'', 'li class=\'form-check ', $content);
		$content = str_replace('<input ', '<input style=\'margin-top:-2px\' ', $content);
		$content = str_replace('<label ', '<label class=\'form-check-label\' ', $content);
	}

	if($field["type"] == 'radio') {
		$content = str_replace('li class=\'', 'li class=\'radio ', $content);
		$content = str_replace('<input ', '<input style=\'margin-left:1px;\' ', $content);
	}

	return $content;

} // End bootstrap_styles_for_gravityforms_fields()
add_filter("gform_submit_button", "form_submit_button", 10, 2);
function form_submit_button($button, $form){
    return "<button class='button btn btn-primary' id='gform_submit_button_{$form["id"]}'><span>Submit</span></button>";
}

Thanks for this. As a minor improvement, you could preserve custom submit button labels by changing the gform_submit_button filter:
add_filter("gform_submit_button", "form_submit_button", 10, 2); function form_submit_button($button, $form){ return "<button class='button btn btn-primary' id='gform_submit_button_{$form["id"]}'><span>" . $form['button']['text'] . "</span></button>"; }