The grid display from the SimpleLightbox homepage is just for demo purpose, is not included in the library. An important feature that is missing is the support for JSON input, which is important for the possibility to have better control on what information is selected for each image.
About the captions of the Gallery library, it can be solved. In order for the library to work, you need to manually add the lightbox template (like it is done for the Bootstrap modal for example), so the layout can be modified.
In conclusion, I think the best is to go with the Gallery library.