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Before you can install a "desktop environment" (a GUI) on your computer, you'll need to install the base for it. Type sudo pacman -S xorg-server xorg-xinit xorg-server-utils into the command line and press Enter. When prompted, type in y and press Enter. Type in sudo pacman -S xorg-twm xorg-xclock xterm and press Enter. When prompted, type in your root password and press Enter, then type in y and press Enter. This may take a few minutes since the packages are being downloaded onto your computer. Type in startx and press Enter. Doing so will open the X Window System command line, from which point you can install the GNOME GUI.
Enter the X Window download command. Confirm the download. Enter the command to install desktop features. Enter your password, then confirm the download. Wait for the installation to complete. Start the X Window System.