no. you will be limited to whatever is on Intels website CPU support list for that motherboard. Also i think the max FSB on that motherboard would be 800mhz, so the cpu will have to have its FSB rated for 800 also. http://www.intel.com/support/motherboards/desktop/d915g... theres the link for cpu support list.
Nearly any board should fit. Only thing is your tower isn't as tall as others so you'll need any mATX board. They are common however so this shouldn't be a problem.
You'll also need a board that has onboard video, or a video card. (GPU) Depending on the one you pick you might need a new PSU as well. And because the OS you have on there is looking for the gateway bios, you'll need a new copy of windows as well.