add_filter( 'gform_save_field_value_3_16', 'change_field_to_uppercase', 10, 3 ); //change formID and fieldID (the _3 and _15) to relevant targets 
// and remove the field id ( _15 ) if you want it to apply to all fields on the form and remove the form id ( _3 ) if you want it to apply to all forms

function change_field_to_uppercase( $value, $entry, $field ){
	if ( $field->get_input_type() === 'text' ) {
		$value = strtoupper( $value );
	}
	return $value;
}