The GNOME Project is a free and open source desktop and computing platform for open platforms like Linux that strives to be an easy and elegant way to use your computer. GNOME software is developed openly and ethically by both individual contributors and corporate partners, and is distributed under the GNU General Public License.
OpenGL is the only other open source alternative. Vulkan is developed by the folks who created and manage the OpenGL specification. (https://www.khronos.org/) To shorten the list of differences I will specify one. OpenGL is ancient and can not compete on modern hardware. Vulkan is the new hotness that everyone in the future will support.
There are a many reasons that more fully answer your question but this is the short/fast.
I think it's more that opengl is a higher level API that offers less granular control in exchange for easier cross platform support.