Did you take a look at http://golang.org/doc/gdb? I can see how a debugger is nice when exploring a new code base, but when debugging I find it’s usually enough to understand what a given data structure looks like at some point in the code, and besides the bug is probably not happening on my own system anyway but somewhere else out there - hence the STTRACE stuff. 
As for why Go and not Java or Scala, first of all I think Go is a much much nicer language, and Syncthing in Python?.