Alfresco's CIFS interface should suffice for GSA to index at it normally would. CIFS (Common Internet File System) protocol is supported by the repository be default, and Alfresco appears like a regular Windows box on the network. Spaces within Alfresco can appear as Windows shares, and therefore various application, including GSA, can walk the content items (files) within the spaces and work with these files as they would on a regular file system.
Hope that helps.
–Sumer