Ensure name input is letters only

<?php
// http://www.gravityhelp.com/forums/topic/name-field-1#post-169715
// change the 350 here to your form ID
add_filter('gform_validation_350', 'force_alpha_name', 10, 2);
function force_alpha_name($validation_result){

    $form = $validation_result['form'];
        // In my form, the name input is field 1, so first name is 1.3 and last is 1.6
        // I put them together here to test them both at the same time
        $names = rgpost('input_1_3') . rgpost('input_1_6');
        if(!ctype_alpha($names)) {
        $form['fields'][0]['failed_validation'] = true;
        $form['fields'][0]['validation_message'] = 'Sorry, please use only letters in your name.';
        $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;
}