Command line to return to the GUI after Ctrl-Alt-F1?

chvt allows you to change your virtual terminal.

From man chvt:

The command chvt N makes /dev/ttyN the foreground terminal. (The corresponding screen is created if it did not exist yet. To get rid of unused VTs, use deallocvt(1).) The key combination (Ctrl-)LeftAlt-FN (with N in the range 1-12) usually has a similar effect.

Use the w command to find out which tty your X session uses, so you won't need to guess which Fn to press.

You can simply use Alt+F1 to return to your desktop.