To me, I just don't like it because it sounds like someone that talks way too much. :) I LOVE Gabriella, but for some reason, Gabby just puts me off. However, Gabbi or Gabi don't seem to bother me as much (looking at them), but I think it's the sound I don't like. Other nn's for Gabriella could be:
Briel
Ella
Elle
Bri
WDYT?