The GPU profiler is similar to the CPU profiler with the various contributions to rendering time shown as a hierarchy in the bottom panel. Selecting an item from the hierarchy will show a breakdown in the panel to the right.
Please note that on the Mac, GPU profiling is only available under OSX 10.9 Mavericks and later versions.
Is something described here not working as you expect it to? It might be a Known Issue. Please check with the Issue Tracker at issuetracker.unity3d.com.