Ctrl+L redraws the terminal; it doesn't clear it. If you're in a full-screen app like less or vim, the Ctrl-L command is what you use to redraw a corrupted screen. In vim with color syntax highlighting, for example, you can use ctrl_l to update the colors if you scroll a long distance and vim gets confused by matching quotes or brackets or similar.
Just for reference if someone searches and finds this... If you need to clear the scroll-back buffer, either set your buffer to 0 lines or close the window and reopen it. Or "while true; do print; done" and then interrupt with ctrl+c when you've output enough lines to blow the buffer. The scroll buffer is application dependent, so while the given solution works for Gnome terminal, it won't work for really any other terminal device.