Synaptic, apt-get, aptitude, none of them will install (with automatically downloaded dependencies) local packages for you.
gdebi is a simple tool to install deb files. It lets you install local [Debian] packages resolving [downloading, if needed] and installing its dependencies.
Gnome & KDE versions available for Ubuntu:
gdebi: simple tool to install deb files - GNOME GUISo there you have it. Install gdebi from apt-get, Synaptic, etc. and proceed with your local installs with gdebi. GUI or command line (man gdebi).
gdebi-core: simple tool to install deb files
gdebi-kde: simple tool to install deb files - KDE GUI
Distro: XUbuntu 12.10