You should be able to use OpenSSL for your purpose:
echo | openssl s_client -showcerts -servername gnupg.org -connect gnupg.org:443 2>/dev/null | openssl x509 -inform pem -noout -textThat command connects to the desired website and pipes the certificate in PEM format on to another openssl command that reads and parses the details.
(Note that "redundant"-servername parameter is necessary to make openssl do a request with SNI support.)
