You'll need to edit the GoogleDocs4MW.class.php file. In the $output section (bottom) you'll need to replace the last two lines above the return $output;.
//'" src="http://spreadsheets.google.com/pub?key=' . $key .
'" src="https://docs.google.com/spreadsheets/d/' . $key .
//'&output=html&widget=true"></iframe>';
'/htmlembed?widget=true"></iframe>';
The old format no longer works: http://spreadsheets.google.com/pub?key= $your_doc_key &output=html&widget=true
The purpose of the change is to use the new format required by Google: https://docs.google.com/spreadsheets/d/ $your_doc_key /htmlembed?widget=true