SiliconWizard, OpenVG is a nice library, but GUI requires user interaction, do you know any OpenVG mouse procedures ? At least mouseclick on graphical element ?
As I said, one of the benefits I put forward for using OpenVG is that it woudn't require any desktop environment, and not even X11. So that would be for someone willing a very lightweight solution (and thus very fast power-on to ready-to-go interface), and obviously who can work without X11 and/or fancier libraries handling user input.
AND, not all UIs with a small SBC require user input with a conventional keyboard and mouse; that could be anything else that you'd handle with your own code, like custom buttons, non-standard joysticks, etc. In this case, you're on your own and will have to figure out how to use the GPIOs (/SPI/I2C...) on the RPi.
Now if you absolutely need to handle regular keyboards and mice, it is possible to use OpenVG without X11, but I think it can also be used under X11, in which case you'd have access to the X11 functions for user input. You'd have to learn a bit about X11. A bit more involved.
Obviously, someone not comfortable with this would probably had better use something more high-level, like at least the SDL library, or GLFW+OpenGL (GLFW may be usable with OpenVG but I'm not sure), or higher level such as Qt/GTK/anything else like this.
For the OP's need, which, let's recall, was "a stopwatch timer triggered by a GPIO", I thought talking about OpenVG and a minimal install of Linux was appropriate, as it was the exact case I was talking about above: just a nice display and handling GPIOs. Other user actions could similarly be handled using buttons and GPIOs.