The choice of a plectrum with which to play gypsy jazz is personal and I am sure most guitarists have boxes of different plectra which they have tried and rejected. There does seem to be a general preference for thick plectra which do not flex at all when they hit the strings. I used to use plectra made by Clayton out of a smooth plastic material which were 1.8mm thick and didn’t flex. They were good and I used them for many years. I had two problems with them – they didn’t last long, half a gig of rhythm guitar, and they have smooth sides so occasionally slip out of my grip when playing a hot gig. I now use the handcrafted Wegen gypsy jazz picks which are even thicker, 3.5mm. and have ribs on both sides that stop rotation in your grip. Fortunately, since they cost 15 Euros each, they have a very long life.