I used to use picamilon quite a bit some years ago, for anti-anxiety purposes. I never noticed any withdrawal effects. It did not seem particularly strong though, just a mild calming effect.
I think a good GABA supplement is the herb
kava kava (Piper methysticum). Most drugs, supplements or herbs that act on the GABA system seem to be subject to tolerance and withdrawal effects to some degree or another, due to these medications decreasing GABA system sensitivity in the long run.
However, kava kava is the very opposite: it seems to
increase GABA system sensitivity over time (see
this thread):
the observed effects of kavapyrones were due to an increase in the number of binding sites
Kavalactones increase GABA receptor density in specific areas of rodent brain (especially hippocampus and amygdala)
I am quite fond of kava, and often take 300 mg of kava root, especially if I have to socialize; kava provides a nice calmness to the mind, which helps buffer the effects of any hectic social environment.
The calming effects of kava take around an hour or so to kick in, and last for around half a day.
Note that kava leaves and stem have been associated with very rare liver damage, but I read that kava root is safe.
Kava is the traditional social drink on Polynesian islands such as Hawaii.