(In reply to comment #0)

A query using the gallery caption attribute should look something like this:

{{#ask: [[Category:Images]] ?The property for captions of individual images
goes here | format=gallery | caption=The caption for the whole gallery goes
here | widths=60px | limit=20}}

That's a missing pipe typo. It should be:

{{#ask: [[Category:Images]] | ?The property for captions of individual images goes here | format=gallery | caption=The caption for the whole gallery goes here | widths=60px | limit=20}}

Do you happen to know if the native MediaWiki gallery supports such a global caption.

Yes, it does:

http://www.mediawiki.org/wiki/Help:Images#Optional_gallery_attributes

caption={caption}: sets a caption on the gallery.

Looks like this is already supported by using SMWs into= parameter.

I haven't been able to find any documentation of that. Can you link?

I just added it to: https://secure.wikimedia.org/wikipedia/mediawiki/wiki/Extension:Semantic_Result_Formats/gallery_format

Wasn't documented before, as quite a few things are not, sadly enough.

SMW parameters
into
is used as global gallery caption.

I tested something similar to this, but no gallery caption was produced:

{{#ask: [[Category:Images]] | ?The property for captions of individual images
goes here | format=gallery | into=The caption for the whole gallery goes
here}}

Typo; should be intro, not into.

Confirmed working. I have updated the documentation with detailed explanation and several examples:

http://www.mediawiki.org/wiki/Extension:Semantic_Result_Formats/gallery_format

Awesome! Thanks :)