It was ever possible to set it in gnome-teak-tool. However, in the latest Gnome 3, it disappears. Is there any other way to set it?
The configuration is the
org.gnome.desktop.wm.preferences titlebar-font under dconf.
You can use
dconf-editor to find the appropriate key entry and set it.
$ gsettings set org.gnome.desktop.wm.preferences titlebar-font 'Sans Bold 10'
To query the settings:
$ gsettings get org.gnome.desktop.wm.preferences titlebar-font