This is a rest API there is no customization of fields.

In graphql you can get what ever fields you want

GraphQl example

curl -i -H "Accept:application/json" -H "Content-Type:application/json" -H -XPOST "https://gorest.co.in/public/v2/graphql" -d '{"query":"query{posts {pageInfo {endCursor startCursor hasNextPage hasPreviousPage} totalCount nodes {id title body}}}"}'