I encountered some issues when I tryed to install YunoHost on Gandi server.
You have to add GPG key to validate Yunohost repository.
But you have to install wget first
apt-get install wget
Then you can get the key
wget -O- http://repo.yunohost.org/yunohost.asc -q | apt-key add - -qq
Now you are ready to install Yunohost.
If there is a problem like “wget: unable to resolve host address” try the following tip
The DNS server seems out of order. Put nameserver 188.8.131.52 to the first line of /etc/resolv.conf