Google Now relies on structured data compatible with schema.org in emails to work. This lets Google understand the fields in your email and construct a Google Now card with the relevant information for the recipient. Only some card types are supported by Google Now.
Schemas in Gmail support both JSON-LD and Microdata and you can use either of them to markup an email. There are two ways of adding semantic markup to your email - you can author the markup yourself or use the Markup Helper to get you started by adding markup to an existing email.