This "Blood,Sweat and Tears" symbol as Pierce described it after the incident is something that I noticed him doing during the introductions before games all year. I'm no expert on symbolism, but he holds his index finger and thumb upside down (like a drop of liquid would look) and extends the other three fingers straight out (1. blood 2. sweat 3. tears).
Most likely, this is an inside gesture the team uses. If that makes it a "gang sign" then okay. What he did towards Horford, from my way of looking at it, was an ill-conceived way of showing the cocky rookie that despite his chest bumping and barking at Pierce that the Celtics' team would ultimately take him down.
Yes, it was a stupid move by the captain due to it's possible connotations, but I think this is sooooo overblown. "If" anyone can definitively link this to an organized street gang, then I will gladly take back my assumptions based on Pierce's explanation. Until then, I say this is BS.