Update Forms Everywhere

gf-update-form-fields.php

function twentytwentyone_gf_add_placeholder() {

	$forms = GFAPI::get_forms();

	foreach ( $forms as $form ) {

		if ( $form['id'] !== 1 ) { // Or Any Form ID
			return;
		}

		$fields = $form['fields'];

		foreach( $fields as $field ) {

			if ( $field->type === 'select' && $field->label === 'State' ) {
				$field->placeholder = 'Choose a State';
				$field->isRequired = true;
			}
		}

		$result = GFAPI::update_form( $form );
	}

	return $result;
}

twentytwentyone_gf_add_placeholder();