I might be missing something obvious, but is there any way to get an insight into why the GSA results for a query are what they are? E.g. Lucene searchers have explain method. Is there anything similar in GSA?
This would be extremely useful when you don't quite understand why you are getting results that you are getting and why the order is what it is.
No. According to all expert reports in enterprise search domain (i.e. Gartner, but not only), Google never explained how it ranks search results in GSA.