<?php
add_action( 'gravityflow_post_webhook', 'sh_action_gravityflow_post_webhook', 10, 4 );
function sh_action_gravityflow_post_webhook( $response, $args, $entry, $current_step ) {
	//Replace step ID and fields for calculation to match your use case.
	if ( $current_step->get_id() == 77 && is_numeric( $entry['3'] ) && is_numeric( $entry['16'] ) ) {
		$entry['14'] = round( $entry['3'] * $entry['16'], 2);
		$result = GFAPI::update_entry( $entry );
	}
}