"The work, which will be published in Éditions Gallimard outside the collection in 1999, will eventually be added to the collection in 2002"

See the references - "outside (of) the collection" seems to be widely used in a similar context.
It depends on the context, but I'm guessing the book will be published in two different editions and the 2002 edition will be part of a collection of related works, such as a "Classics collection", which is something many publishers have.

In the field of book publishing, a collection, or more precisely editorial collection (French: collection éditoriale; Spanish: colección editorial; Italian: collana editoriale; Portuguese: coleção de livros, lit. 'collection of books') is a set of books published by the same publisher, usually written by various authors, grouped under the same title, each book with a particular title. The collective title is the title of the collection, it must be mentioned on each book.[1]

The books that make up an editorial collection can be published in a specific order or not. When each volume in the collection has a serial number, it is called a numbered collection.[1]

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Note added at 46 mins (2021-09-30 02:51:54 GMT)
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"collection" is fine in this context

There's a reference to the term 'uncollected':
"'Uncollected' is a French publishing house or editing company founded in 1992. They publish books on music and film, classical book editions, humourous work and cartoons. At the initial phases City Press was a member of another publishing or editing company, with a group called 'living editions'. In 1988, a literary ghost writer at City Press created the uncollected editions and then became part of a publishing firm in 1991. A literary ghost writer is a subcontracted anonymous author of a text which is signed by someone else, sometimes by a famous person."

(FR: Il y a une reférence au terme 'hors colection':
"Hors collection est une maison d'édition française fondée en 1992. Elle publie des livres sur la musique et le cinéma, des beaux livres, des recueils humoristiques et des bandes dessinées. Au départ collection de Presse de la Cité elle apartient à Place des éditeurs, filiale du groupe Edites Viventi. En 1988 un nègre littéraire aux Presse de la Cité, créé la collecton 'hors collectoin' qui devient une maison d'éditions en 1991. Un nègre littéraire est l'auteur sous-traitant et anonyme d'un texte signé par une autre personne, souvent célèbre.")
https://fr.wikipedia.org/wiki/Hors_collection

If similar to 'hors série'