Great question. Couple of thoughts.
1. I found that sometimes it was a matter of class weight. Adding more classes means your styles override the default ones.
2. You can see what priority the GB styles are enqueued at and make sure yours are enqueue later.
3. You can find the handle for the core GB styles and make them a dependency for your stylesheet to have WP ensure yours loads later.
One or any combination of these should help.
A good place to start would be the core GB plugin file, looking for where it enqueues CSS in the editor 🙂