Usando colunas fictícias para contabilizar as diferentes estruturas e adicionar uma união para juntá-las:
SELECT * FROM (
(SELECT article_id, title, article , NULL as cloudinary_id, NULL as url, user_id, created_on, 'article' as table_name FROM article)
UNION ALL
(SELECT gif_id, title, NULL as article, cloudinary_id , url, user_id, created_on , 'gif' as table_name FROM gif)
) results
ORDER BY created_on ASC
