Qt4 Unable to start gdb 'gdb': No such file or directory

Set in Tools > Options > Debugger > Gdb > Gdb location: /usr/bin/gdb and then F5 will work fine!

Qt4 error exit code 2

Set in Tools > Options > CMake > CMake executable : /usr/bin/qmake-Qt4
and then press the play button. Your project should be compiled now.