Reuseable sections can be saved into Library. Edit one, apply to any pages.
From a user: I have elements I've created which are on every page I've designed with GemPages. I've saved them as custom blocks in my library. But if I make a change to the custom block I have to re-save it and replace it on every page by hand. I want to be able to edit the block I've created directly and have it update every instance of that block automatically.