would search engines actually display anything useful about a gedcom file ?

If by public domain you mean copyrightable then that I think depends - you cannot copyright public info (as is BMD info) nor can you copyright an obvious (possibly only) format for such info. However selection of individuals to form a group might just be copyrightable. Any summary of a will for example would however be copyright of the summariser - a grey area might be the actual reading of a MI which requires judgement.