You can certainly install two Linux versions on one hard drive. First you need to backup all your data since things CAN go wrong.
Next, use the partition editor to make sure your hard disk has room for the new system. How you set it up originally and how you plan to use the new system are very specific to the user so I can't give you any advice there. You can use either Gentoo's partition editor or wait for Ubuntu to launch its version during the install. Be careful not to commit to partition changes unless you really understand the changes its going to make as you could potentially mess up BOTH installations if you make poor choices.
Once you boot the Ubuntu install CD it will ask if you want to run live or install. Probably best to run live at least once to make sure you can use the stuff you need to use - wireless, ethernet, USB drives etc. After testing thoroughly and assuming you really do want to install then just restart and pick the installation option instead of live. Since you say this is a reinstall I'm assuming that won't be a problem. If everything goes well grub should allow you to pick either Ubuntu or Gentoo on bootup.
I can't stress enough - back up the stuff you need before you try this. I've done installations like this many times and I can say from experience that "Murphy is alive and well and living in your computer."