GLES builds were not tested except for emscripten (which provides gles headers). Looks like the gles headers need to be added for glad.
What platform do you want to use GLES in? Linux? Android?
For android, it’s enabled in EGL at https://gitlab.kitware.com/vtk/vtk/-/blob/master/Rendering/OpenGL2/vtkEGLRenderWindow.cxx?ref_type=heads#L442.