ddd is a graphical frontend to gdb and a few other debuggers (like jdb). ddd's UI is straight out of the 90s era of commercial Unices, but I prefer it to gdb since it visualizes source code with breakpoints and the ip and has some nice variable displaying / plotting features (ie it's very easy to visualize a matrix AS a matrix if you're doing some graphics work)

edit: ddd also has a gdb prompt at the bottom, so you really don't lose much by using ddd over gdb unless you can't run an X server

Maybe it's professor specific, but EECS 151, 280 and 281 all stressed using gdb. ddd was mentioned, but most of the debugging examples used gdb. I took these class several years ago though, so it's also possible things have changed since then.