GOverlay acts as an interface for Rivatuner Statistics Server (RTSS) to display the information above the DirectX or OpenGL game.
GOverlay handles the information itself while RTSS handles the rendering on the screen.
RTSS is a free application that used to come with Rivatuner, then bundled with EVGA Precision and MSI Afterburner software (also free softwares) and now is a standalone application that can be downloaded from guru3d.

In RTSS you can configure which applications will show the information on the screen and which ones will not, this is useful because sometimes some FLASH content in the web (like reproducing videos from youtube) can display your GOverlay information, so in this screen, you can add the application so it wont display it there.
In RTSS you can also configure the font, color and size to display the information in the overlay and the position on the screen.
Remember to enable "Start with windows" and "Show on-screen display" to have the overlay working and also It is important to always disable "Show on statistics" because GOverlay display its own