<?php
    // http://www.gravityhelp.com/forums/topic/field-validation
    // http://www.gravityhelp.com/forums/topic/match-fields
    // update the 1 here to your form ID
    add_filter('gform_validation_1', 'matching_field_validation', 10, 2);
    function matching_field_validation($validation_result){

        $form = $validation_result['form'];
        // this compares field 2 to field 3, update these values to match your form
        if (rgpost('input_2') !== rgpost('input_3')) {
            // this is the validation error for input_1
            $form['fields'][0]['failed_validation'] = true;
            $form['fields'][0]['validation_message'] = 'This field must match the next one.';
            // this is the validation error for input_2
            $form['fields'][1]['failed_validation'] = true;
            $form['fields'][1]['validation_message'] = 'Please verify your entries.';
            $validation_result['is_valid'] = false;
        }

        // update the form in the validation result with the form object you modified
        $validation_result['form'] = $form;
        return $validation_result;
    }