Reading the contents of the pdf at Drivethroughrpg, I can only find references to the rules and a table at the back of the book/ pdf.

Do I still need Winds Of Magic or Signs Of Faith to actually use mutations?

What about diseases?

Mutations are listed on page 162+163 in the GM's Guide. And Diseases are listet on pages 164+165 in the GM's Guide.

That's all the information there is. To play with Mutations and Diseases in "WFRP lite", you don't need anything else. If you want to play with mutation and disease cards, you have to buy Winds of Magic and Signs of Faith. But since the GM Guide is a book and doesn't contain any cards at all, this seems natural.

Great to know! Thanks a lot!