Tonal ambiguity

Debussy uses a Gb major key signature. The prelude ends in Gb major and we find several cadences to Gb in the piece. Is the prelude in the key of Gb major?

The answer to this question is not so simple as the piece constantly changes from Gb major to an Eb pentatonic/minor natural. Probably this ambiguity was precisely what the composer intended in this piece and in most of his music.