This is a very generic answer because you don't give any relevant details in your question.
Screen rotation (by a quarter turn) is a well-known feature. Some screens even have sensors that report their orientation to the video card so that it rotates the display accordingly. Its support however depends on both the video card and the video drivers, so it is not always available and may have a cost (sometimes you have to choose between a rotated screen and decent display performance).
Gnome doesn't matter here, except in as much it gives you a dialog box where you can click on some settings. What matters is:
- your operating system (e.g. “Ubuntu 10.04”, “OpenBSD 4.7”);
- your exact video card model (if you don't know,
lspci | grep -i display should give the relevant information);
- what video driver you are using (and whether you're willing to change if necessary). (Under Linux, you often have a choice between an open-source driver and a closed-source driver. Sometimes only one of them can do screen rotation, but it might be unable to do something else like 3D or dual head.)
To see if your current software configuration supports screen rotation, run xrandr with no parameters. If it prints a line like DVI-0 connected 1920x1200+0+0 (normal left inverted right x axis y axis) 517mm x 323mm, your system supports rotations and mirroring. If you see only (normal), you're out of luck with your current driver version and hardware.
