Some people have been having some problems with Grandstream gateways, choppy sound, poor quality voice... just like you are experiencing...
I can tell you that Voip has very good quality voice(depending on codec used and avaible bandwith) of corse everything needs a bit of tweaking... maybe thats the problem with your gateway...
You can always get a better gateway. People been using Patton alot, saying voice quality is very good, i use here a Linksys SPA3102, also works very good...
Cheers