It can give you some shortness in breathe...It just depends on dose and the individual IMO...There will definately be some "flushing" that can be felt which is dose dependant...personally i wouldn't take GABA, im sure there are safer things out there to take. If i remember correctly i think GABA has a relation to your dopamine levels, and this can influence testosterone levels/sensitivity/production. Google for more info i guess.
Btw im just speaking from personal experience as 2 years back i use to use GABA to increase HGH, personally i dont think it gives the HGH increase that is claimed, and i was using 5g shortly before bedtime, and i used it approximately for 3months pretty continually if i remember correctly, i definately noticed the slight flushing, but i didn't really experience shortness in breathe. it definately helped to sleep, but i sleep like a rock without supplements as it is. If your looking for a rise in HGH levels and/or to help with sleep, definately get yourself some L-glycine powder.
NOW foods manufacture some good L-glycine powder that can be taken with a small amount of water, (again speaking from experience, i use l-glycine when i feel like it) and the rise in HGH is definately better than GABA powder. Sorry for the long post my friend, hope my input helps a bit.