gdb fails with "Unable to find Mach task port for process-id" error

In Snow Leopard and later Mac OS versions, it isn't enough to codesign the gdb executable.

You have to follow this guide to make it work: http://www.opensource.apple.com/source/lldb/lldb-69/docs/code-signing.txt

The guide explains how to do it for lldb, but the process is exactly the same for gdb.