A very common chord substitution in Gypsy Jazz is to replace a dominant 7th chord by a diminished 7th chord a half tone higher.
So for example, in A minor, instead of playing E7 -> Am, you could play Fdim7 -> Am.

Here is the logic behind that substitution.
The E7 chord is (E, G#, B, D); but in a minor context, this chord is often altered by adding a b9; so the chord becomes (E, F, G#, B, D).
In the diagram below you’ll see the E7 and E7(b9) arpeggios the way a Gypsy Jazz guitarist would typically play them.

But remember that Django Reinhardt only had the pointing finger and the middle finger of his left hand available for fretting, so to make that chord easier to play he simply dropped the root (that the bassist or another guitarist would probably play anyway).
And so he ended up with (F, G#, B, D) or equivalently (F, Ab, Cb, Ebb) which is Fdim7!
This is shown at the bottom of the diagram above.