Use the manual installation as described in the Wiki. The automatic one is strange.

On Gentoo, at least the following packages are required:

• net-libs/libosmocore
• 

git clone https://github.com/ptrkrysik/gr-gsm.git
cd gr-gsm
mkdir build
cd build
cmake ..
make
sudo make install
sudo ldconfig
wget -O /etc/udev/rules.d/rtl-sdr.rules http://git.osmocom.org/rtl-sdr/plain/rtl-sdr.rules

Scanning the Channels does not work :

sudo sysctl kernel.shmmni=32000
for b in  P-GSM DCS1800 PCS1900 E-GSM R-GSM GSM450 GSM480 GSM850 ; do echo $b ; PYTHONPATH=/usr/local/lib64/python2.7/site-packages python2 airprobe_rtlsdr_scanner.py -g 30 -b $b -p 0 -v --speed 1 ; done

To Sniff TN0:

wireshark -k -Y '!icmp && gsmtap' -i lo
PYTHONPATH=/usr/local/lib64/python2.7/site-packages gnuradio-companion ../apps/airprobe_rtlsdr.grc