How can I format a drive using the GPT format on OS X? I don't see this option in Disk Tools.
Assuming you mean Disk Utility, you have to select the drive itself (as opposed to an individual partition)*, and then you will see a Partition tab. Then on that tab, select the partition layout you want, and at the bottom click 'Options...' — you'll be able to select GUID Partition Table from there.
( * Apologies if this is patronising, but just to be 100% clear, in order to change the partition scheme you'll have to erase the entire drive. You can't change it for just one partition. )