Register field as a list of strings
The below code registers a field called listOfStrings that returns a list of strings as the result:
add_action( 'graphql_register_types', function() {
	register_graphql_field( 'RootQuery', 'listOfStrings', [
		'type' => [ 'list_of' => 'String' ],
		'resolve' => function() {
			return [
				'String One',
				'String Two'
			];
		}
	] );
} );This field can now be queried:
{
  listOfStrings
}