The interfaces of Skype and Gizmo and very similar in some places, and very different in others. In terms of basic layout, both Skype and Gizmo have many similarities. When either program is loaded up you are presented with a central screen which provides numerous options. However, while Skype tries to incorporate almost everything into the central interface screen, Gizmo sorts things into tabs.
For example, in Skype making a call doesn't require navigating to a new part of the interface but rather introduces a new element on the existing interface. This isn't always less confusing, however. For example, finding the number pad while on a call is confusing in Skype because there are actually two number pads - one for making a new call and one for dialing numbers on your existing call. Gizmo simply has a single dial tab, which is more intuitive.
Skype looks much slicker than Gizmo, which uses a fairly ugly interface which brings back memories of Windows 98. Skype makes better use of graphical elements to make the interface appear more professional, but this also appears to have an impact on the memory the program uses. Gizmo uses around 15,000kb on my system, while Skype uses closer to 25,000kb. This victory rings a little hollow, however - realistically, neither takes up enough memory to cause a significant slowdown even on a netbook.