This is perfectly normal with the ports system, ports doesn't just install a program, it downloads the source code and compiles it. How long this will take depends on how large the program is, and how fast your machine is. Gnome is of course 'very big', the best way to install such programs is via a binary package or a PBI, although in this case a PBI does not exist.
If you look at the 'gnome' section of the FreeBSD port list (http://www.freebsd.org/ports/gnome.html
) you'll see a number of ports. Most of them have 'package' links which allow you to download and install the binary (meaning you avoid compilation). It'd take you a while this way though because of the large number of dependencies, the best way is to use the 'remote fetch' option of pkg_add (-r) like this (as root):
pkg_add -r gnome2-2.10.2
Hope that helps