No, not all metastrings. Just the orphaned ones that are not used anymore.

Thank's Juho.

I try to read gc plugin code and i find the query used to delete metastring that don't have metadatas and annotations.

SELECT * FROM {$CONFIG->dbprefix}metastrings WHERE
        (
            (id NOT IN (SELECT name_id FROM {$CONFIG->dbprefix}metadata)) AND
            (id NOT IN (SELECT value_id FROM {$CONFIG->dbprefix}metadata)) AND
            (id NOT IN (SELECT name_id FROM {$CONFIG->dbprefix}annotations)) AND
            (id NOT IN (SELECT value_id FROM {$CONFIG->dbprefix}annotations))
        )

Never,It will delete only those which does not have metadatas and annotations as said by @ Riadh Hamdi