Same problem here...I have a Gabriella and people insist on callling her Gabby, drives me absolutely crazy. She is most definitely NOT a Gabby and Gabriella suits her perfectly. I wanted to shorten to Brie as well (funny, don't think its common, so was suprised to see someone else with the same idea), but it never stuck. I am constantly correcting people and now my daughter does as well. The only nicknames that have stuck for a while but have faded a bit are Gabba (when she was in a toddler day care room none of the kids could say her name

) and then a teacher called be Gabs.
I try and take my cue from parents at school, if they call their child by a nickname/shortened name, then I do the same. There is 1 parent at my daughter's school that insists on calling my daughter Gabby, even after 4 years of NEVER hearing me say that and correcting her numerous times. I started called her son Andrew, Andy and she quickly corrected me..."oh we don't call him that"....well get a clue lady we don't call our child Gabby!