Recently, i have installed FreeBSD 5.3 on my computer. I then proceeded to install the latest version of Gnome. I went to the FreeBSD handbook and looked up how to set Gnome as my default window manager, and for some reason it did not work. I was wondering what exactly the command is to do set gnome as default.

It will be easier to answer if we know what you did do.

I'm not sure I understand what you mean... anyway, I hope I do
So, if you want to start gnome when you type startx on your prompt, try to edit your ~/.xinitrc by writing there on the first line gnome-session. This will start gnome when you're trying to start the graphical interface (when you type startx).
In case that you want to start graphical automaticly when you start your computer, you should edit your /etc/tyys file and change the line that starts with ttyv8. On my computer I use KDE and it looks like this:
ttyv8 "/usr/local/bin/kdm" xterm on secure
On your computer, I gues that you want to start gdm... then replace "/usr/local/bin/kdm" with the absolute location of your gdm file. You can find it by typing "whereis gdm"
I hope this helped...
Bye !

i get the feeling i'm following Serg
once again:
try freebsd 5.4 beta. it has gnome 2.10, and i have yet to get hit by a bug.
pre-compiled and the ports are really smooth.
luck -O.

edit: i was assuming he was a compile-it-yourself junkie. if you want a fast easy install, use 5.4 and just follow the install prompts.
if you glitch there, do this:
# echo "exec gnome-session" > ~/.xinitrc
for root, and for each User as well.
-O.

It seems that FreeBSD-5.4 is the answer for averything... :P