@jeko's answer is good, here are a few additional ways to get the response body:

let-values

(use-modules
  (srfi srfi-11)
  (web client))

(let-values
  (((response response-body) (http-request "http://www.gnu.org")))
  (display response-body))

define-values

(use-modules
  (srfi srfi-244)
  (web client))

(define-values (response response-body) (http-request "http://www.gnu.org"))
(display response-body)