Please quantify what you mean by disappointing. What slowed down?
In any case, I don't think your motherboard is designed to overclock easily. If there are no options within the BIOS, you may have to open up your system and look for any OC jumpers. Alternatively, you can check with Gateway itself over what's possible.
Also, your slowdowns may be due to incompatibilities between your mobo and the newer processor. Are there any firmware updates available for your desktop? Maybe you can try installing that and see if the update improves your system performance.