The earliest citation seems to be in Jack Chambers’ Davis biography Milestones.  Chambers (p. 240) appears to cite a 1973 reissue of 'Round Midnight as the source of the Gil Evans information (or misinformation):  "Evans's role was not credited publicly until this title was reissued in 1973."  'Round Midnight was included on the compilation called "Basic Miles: The Classic Performances of Miles Davis," released in 1973, with liner notes by Mort Goode.  Chambers includes these liner notes in his bibliography.

However, Goode's liner notes make no mention of Evans arranging 'Round Midnight; instead, the "Discography" following the notes incorrectly lists the Gil Evans orchestra as being on 'Round Midnight ("With Orchestra under the direction of Gil Evans" -- of course there's no orchestra on the recording).  We can't find any mention of Evans arranging the tune.  And virtually all of the material in the arrangement can be traced back to the 1948 Dizzy Gillespie big band arrangement of “’Round Midnight”.

Lewis Porter, in his Coltrane biography (pp. 102-103), notes that the arrangement is "said to be" by Evans, and quotes George Avakian to the effect that it was certainly possible, but Evans definitely wasn't in the studio.   It looks like we should have included or cited this quote in the Coltrane Reference discography, rather than simply listing Evans as the arranger, without citing a source. 

On a possibly related note, Gil Evans definitely made some sort of contribution to "All Blues."  This was reported by Ralph J. Gleason in 1959, before Kind of Blue was released ("The Miles Davis Sextet All Play Miles' Way," San Francisco Sunday Chronicle, June 7, 1959, Datebook, p. 23): " 'All Blues,’ a remarkable waltz written by Davis, took almost six months to compose slowly, part by part, at the piano at Davis' home in New York. Finally, when he had it, he brought it to Gil Evans, his close friend with whom he has collaborated on several LPs, to get it ready for recording. 'I wrote it in 4/4,' Miles says, 'but when we got it to the studio, it hit me that it should be 3/4. I hadn't thought of it like that before but it was exactly right.' "  Gleason got this information directly from Davis (the sextet was appearing at the Blackhawk), and it seems pretty reliable.

(7/6/08 CD)