Any time I've needed to learn a new stlye of music, I've sat with the recordings , muted all the strings with my left hand, and listed to the part while strumming the muted strings. It kinda turns the guitar into a percussion instrument, but it works every time. It also helps to know how the music was performed and from what era' it came from. Gypsy Jazz ,and more specificly Django Reinhhart's "Hot Club" era was in the 30's-40's. The played in cafe's and theatres w/o amplification so they used alot of powerfull downstrokes when playing chords. Also, the "swing" beat was king back in those days, so learning that beat will help tremendously. A basic Swing beat can be;
One-and'a-Two-and'a-Three-and'a-Four
When counting it out, place the downbeats on the "and'a"' . So count it out loud, and slap your hand against your knee on the "and'a" . It'll sound stupid at first ,especially slow, so do it while no-ones's lookin', ;) . As you speed up the tempo however, the beat will hopefully make complete sense. Now pick up the guitar, and try playing a chord on the offbeats (one-two-ect) while keeping time with your foot.
As they had no amps,and had to be heard over the other instruments,AND the audience, they used alot of powerfull downstrokes to project the sound. Common thinking nowadays is that they used light guaged strings with thick tortiseshell picks (some guys use the Dunlop Stubby 3.0mm's today!!!). The French-made guitars they used had a longer scale length so the tension was still reasonable with the 0.10 guaged strings they used. Some of these guitars had HUGE "D" shaped soundholes for projection, although most pic's of Django I've seen has him playing a small-holed Maccafari.
Check out some of the Jazz lessions here and look for swing styled stuff, they can probably explain it better anyway.
Hope this has helped more that it's hurt!!!
Ian