I am using gcc 4.7 and gcc 4.8 together with QtCreator 2.7.1.
The problem is that when I use gcc 4.8, I am unable to see the values of my variables
In my case, it was what HostileFork described: ubuntu 13.10 + Qt Creator 2.8
This bug has been fixed in Qt Creator 3, not yet released.
To solve this without recompiling GDB, just use the release candidate of Qt Creator 3.
There are builds available on Qt project website or a PPA (ppa:p12/qtcreator) with last git version but build is broken as of today.