is this possible?
How do I specify the key to use.
As long as this is handled by the particular GVfs backend, this cannot be set up from Tux Commander directly. Private keys within SFTP backend are handled via ssh-agent running in your session. That's gnome-keyring (and possibly KWallet in the futuer) in modern desktop environments. Otherwise you'll need to run other ssh-agent for the whole desktop session (backends run out of process, spawned via dbus).
gvfs-bin was not installed.
now it works with the settings in ~/.ssh/config