No, the OS loader (or boot loader) is called GRUB (Grand Unified Boot Loader). Gnome is one of the two most widely used Desktop Environments(DE) available for Linux. The task of a DE is to control your Graphical User Interface (GUI). If you install the default Ubuntu distribution you will get Gnome.
While the Linux kernel itself is quite lightweight the two major desktop environments for Linux (Gnome and KDE) are (opposite of what you may have heard) actually much heavier than both win2k and winxp. There is a lightweight GUI called XFCE which should be able to run fluently on your old p2 machine. You can install it by entering sudo apt-get install xubuntu-desktop in the terminal. You should be aware that Linux based servers are run without an GUI most of the time because in Linux, The GUI (X server) and the kernel is two separate entities, this is opposed to windows there the GUI is deeply integrated into the kernel. This gives Linux the advantage of being able to run a server without having to waste resources on a GUI you don't need anyway.If you (like most new linux users) find the command line frightening there's no shame in starting out with a lightweight GUI (like XFCE) and then gradually getting used to the command line. Over time you will learn to love the command line and realize that it can accomplish a lot of tasks a lot faster than a GUI
If you are worried that linux hasn't detected both of your cpus open the terminal and run this command:
cat /proc/cpuinfo | grep processor
If both cpus are detectet it should display the following output:
processor : 0
processor : 1
If you only see one line, ubuntu has most probably not detectet your second cpu.
In any case, I doubt enabling the second cpu will give you any noticeable performance improvement in Gnome. Having two CPU's doesn't make your computer twice as fast, it only gives you the ability to run two separate tasks simultaneously (like encoding a video and play a game). This have many advantages (especially for server use) since you can run many processes at the same time without the CPU having to waste any clock cycles on dividing the CPU time between processes, but most single processes is incapable of using both CPU's at the same time, this includes Gnome. So I'm pretty sure Gnome will run at about the same speed whether you've got to cpus or not.