It would be great, if it was possible to implement an interface (say: ISummaryInfo) that would allow me to add custom build information (like information from GebConfig.groovy) to my summary screen.

I invision that I could have a custom class implementing ISummaryInfo and have a configuration option like com.athaydes.spockframework.report.ISummaryInfo=my.package.MySummaryInfo

The summary info could just be:

interface ISummaryInfo {
     Map getSummaryInfo()
}

and in TemplateReportAggregator this could be added:

        engine.createTemplate( template )
                .make( [ data          : allData,
                         'utils'       : Utils,
                         'fmt'         : new StringFormatHelper(),
                         summaryInfo: summaryInfo ?: [:],
                         projectName   : projectName,
                         projectVersion: projectVersion ] )
                .toString()

The setup of the summaryInfo should happen where the configuration is normally loaded, and it should be injected into TemplateReportAggregator

Let me know what you think.