We still have to add the source to sources.list then do your aptitude command
aptitude update
you might need apt-file not sure.
first install librtl0
https://packages.debian.org/sid/rtl-sdr
Pick which architecture you have and open that folder to get the sources
https://packages.debian.org/unstable/hamradio/gqrx-sdr
pick a ftp server closest to you
This is assuming you have debian 7.5.0 LXDE
You might get a boot warning saying similar to gqrx invalid rules. I am investigating how to fix it.