You can check the whole "schema" of the response given by Google using this gist.
You'll see that each entry has the following items:
- title
- link
- author
- publishedDate
- contentSnippet
- content
- categories
So, unfortuately for you the guid is missing. You'll have to use content (or contentSnippet) instead of description and publishedDate instead of pubDate. If you really need the guidcheck out the alternative offred by Superfeedr (I created Superfeedr!).