How to specify which GDB (with the full path) I can use in XCode on MacOS? Thank you.
You can set the PBXGDBPath user default for Xcode to specify a different version of GDB for Xcode to use.
defaults write com.apple.Xcode PBXGDBPath <path/to/your/gdb>
http://developer.apple.com/documentation/DeveloperTools/Reference/XcodeUserDefaultRef/100-Xcode_User_Defaults/UserDefaultRef.html#//apple_ref/doc/uid/TP40005535-CH3-SW54