gitosis is replaced by gitolite in debian wheezy.
But the easiest way for to continue your work after an update would be to just install gitosis from source or a debian packet.
Is this possible on wheezy?
You can download the packet from http://packages.debian.org/squeeze/gitosis
cd /tmp
wget http://ftp.de.debian.org/debian/pool/main/g/gitosis/gitosis_0.2+20090917-11_all.deb
#install all dependencies:
apt-get install adduser git-core openssh-server python python-setuptools python-support sudo
dpkg -i gitosis_0.2+20090917-11_all.deb