There are (at least) two packages in Debian which provide tools to manage startup applications.
The first is gnome-tweak-tool (now gnome-tweaks — look for “Tweaks” in the software manager); its “Startup Applications” tab allows you to manage your startup applications in your desktop environment.
The second is systemd-ui; it shows all the configured systemd units and jobs on your system, and allows you to start, stop, restart and reload units. It also displays the description and dependencies of each unit (but not the links to the documentation which may be given in the unit). It doesn’t seem to allow enabling and disabling units though, which is probably what you’re after.