gw-capitilize-specific-field.php

<?php

/**
* Save field in uppercase when form is submitted
*/

add_action( 'gform_pre_submission_123', 'gw_capitalize_submitted_data' ); // Replace 123 with your form ID
function gw_capitalize_submitted_data( $form ) {
	
	$input_key           = sprintf( 'input_%s', 1 ); // Replace 1 with your field ID
	$_POST[ $input_key ] = strtoupper( rgpost( $input_key ) );

	return $form;
}