User meta query

// WP_User_Query arguments
$args = array(
	'meta_query'     => array(
		'relation' => 'AND',
		array(
			'key'     => 'gym_id',
			'value'   => '12345',
			'compare' => '=',
		),
	),
	'fields'         => array( 'id' ),
);

// The User Query
$user_query = new WP_User_Query( $args );