The gallery macro will be used like this (example for XWiki 2.0 syntax):
The content will be parsed using the syntax of the document where the macro is inserted. All the images found in the provided content will be displayed. Images are detected on the client side, so they can be generated using the HTML macro.
The gallery macro won't have any parameters and its content will be mandatory. Also, the gallery macro won't be supported in an in-line context.