The main part is that you need to have your custom Python and gdb build. The Ubuntu Python version has compiler optimizations removed, which makes it difficult to use. You'll also need to install all your package dependencies using the virtual environment that you've setup.
1. Build your own custom Python version. Instructions are here.
2. If you're using Ubuntu 14.04, you need to compile your own gdb too, since the one that comes is linked to Python 3:
a. Download the source from https://launchpad.net/ubuntu/+source/gdb/7.6.1-0ubuntu3.
b. sudo apt-get install libncurses5-dev texinfo
c. ./configure --with-python=python2 --prefix=/home/rhu/special (or wherever you intend to do make install).
d. make install
3. Locate the libpython.py from your Python install.
a. sudo /home/rhu/special/bin/gdb -p <python process>
You should now be able to do py-up, py-down, py-list, py-locals to step through the various call stacks.