Clicking a device on the file systems tab should launch baobab
As per gnome-system-monitor's documentation:
To check how much space is available on a disk, go to the File Systems tab and look at the Used and Available columns. [...] If you don't have much disk space left, there are a few things you can do to try and free up some space. One is to delete unwanted files manually. You can use the Disk Usage Analyzer application to tell you which files and folders are taking up the most space. [...]
In my humble opinion, double-clicking the device should launch baobab, if it is installed. If baobab is not installed, then xdg-open the folder. There's possibly room for a right-click, context menu, with some form of "Open with...".