I used to think that the answer to this was a very clear 'NO'. Jazz is jazz, and folk is folk. Fans of one type often hate the other type. They sound different.
However, since I started reading the Mudcat posts it seems that what the music sounds like isn't part of what defines it as folk. People all have their own definitions of what constitutes folk music, but the only thing that the hard liners accept is the '1954 definition'. To my mind, the particular case of Gypsy Jazz fits the 1954 definition very well.
"Folk music is the product of a musical tradition that has been evolved through the process of oral transmission."
Now when Django was playing, it was not 'folk music' as such, it was rather an interpretation of the pop music of the day. It certainly incorporates scales and arpeggios that Django had learned from his extended family, and in particular, the right hand technique unlike one found outside gypsy culture. But at that point it was a new development, rather than a tradition.
Now, however, it is some 70 or 80 years later, and Django's compositions and interpretations have definitely been handed down through a process of aural transmission – I say that rather than 'oral', as the music is mainly instrumental.
Long after the jazz ceased to be popular music, one Ian Cruikshank, fascinated by the music, but having no way of learning more about it, decided to go to France to see if anyone still played that way. He had the idea that since Django was a gypsy musician, it might have been handed down. He discovered that yes, it had, and the various families were still playing it and teaching it to their children in turn.
The 1954 definition goes into more detail -
The factors that shape the tradition are:
(i) continuity which links the present with the past;
Obviously there, and
(ii) variation which springs from the creative impulse of the individual or the group;
Variations are spontaneously composed each time a tune is played.
(iv) selection by the community, which determines the form or forms in which the music survives … The term can be applied to music that has been evolved from rudimentary beginnings by a community uninfluenced by popular and art music and it can likewise be applied to music which has originated with an individual composer and has subsequently been absorbed into the unwritten living tradition of a community … The term does not cover composed popular music that has been taken over ready-made by a community and remains unchanged, for it is the re-fashioning and re-creation of the music by the community that gives it its folk character.
The community in question being the various gypsy families. They have certainly re-fashioned it. The chord voicings the scales, the ornamentation and the right hand technique are all particular to the gypsy way of playing. The individual composers of most of the repertoire are known, but that apparently is no bar to something becoming folk music.
There's certainly been a resurgence of interest in the last ten years, I'd say worldwide, and there are now even 'sessions' where people get together to play this type of music. Not very many, like, (there's currently eight in the UK that I know of), but even five years ago I knew of none. It uses acoustic guitars, fiddles accordions – instrumentation folk musicians are used to, rather than trumpets and saxophones, which are somewhat on the loud side for pub sessions. And it's certainly an older tradition than the 'bloke with open tuned guitar' tradition that started in London in the 60s, and which is now all pervasive.
It is counterintuitive to me to think of anything in the 'jazz' compartment as fitting into the 'folk' compartment. I (perhaps naively) tend to think of musical styles as being defined by how they sound. But going on the '1954' definition, - what has come to be known as 'gypsy jazz' might not have been definable as folk music when Django started it, but several generations later, I think it is.