What version are you running?
review board 2.5.13.1 , gitlab version 8.5.4
What's the URL of the page containing the problem?
reviewboardurl/admin/db/scmtools/repository/add/ 

What steps will reproduce the problem?

at admin panel "add repository"
hosting service "GITLAB"
url http://gitlaburl
save -> gives error 502


What is the expected output? What do you see instead?
error logs : 

HostingServiceError: Unexpected HTTP error 502.

2017-07-13 13:14:26,852 - ERROR -  - root - Unexpected HTTP error when linking GitLab account for gauravs: HTTP Error 502: Bad Gateway

Traceback (most recent call last):

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/gitlab.py", line 245, in authorize

    'password': password,

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/service.py", line 383, in json_post

    return self._do_json_method(self.http_post, args, kwargs)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/service.py", line 412, in _do_json_method

    data, headers = method(*args, kwargs)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/service.py", line 245, in http_post

    method='POST', kwargs)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/service.py", line 293, in http_request

    response = urlopen(request)

  File "/usr/lib/python2.7/urllib2.py", line 154, in urlopen

    return opener.open(url, data, timeout)

  File "/usr/lib/python2.7/urllib2.py", line 435, in open

    response = meth(req, response)

  File "/usr/lib/python2.7/urllib2.py", line 548, in http_response

    'http', request, response, code, msg, hdrs)

  File "/usr/lib/python2.7/urllib2.py", line 473, in error

    return self._call_chain(args)

  File "/usr/lib/python2.7/urllib2.py", line 407, in _call_chain

    result = func(args)

  File "/usr/lib/python2.7/urllib2.py", line 556, in http_error_default

    raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)

HTTPError: HTTP Error 502: Bad Gateway

2017-07-13 13:14:26,856 - ERROR -  - root - Unknown error linking hosting account ID=None for hosting service='gitlab', username=u'gauravs', LocalSite=None

Traceback (most recent call last):

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/forms.py", line 348, in save

extra_authorize_kwargs)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/gitlab.py", line 261, in authorize

    % e.code)

HostingServiceError: Unexpected HTTP error 502.

2017-07-13 13:14:26,857 - ERROR -  - root - Unexpected error when linking hosting service account on gitlab: Unexpected HTTP error 502.

Traceback (most recent call last):

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/scmtools/forms.py", line 669, in _clean_hosting_info

    force_authorize=force_authorize)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/forms.py", line 348, in save

*extra_authorize_kwargs)

  File "/usr/local/lib/python2.7/dist-packages/reviewboard/hostingsvcs/gitlab.py", line 261, in authorize

    % e.code)

HostingServiceError: Unexpected HTTP error 502.

What operating system are you using? What browser?
ubuntu 16.04 LTS
Please provide any additional information below.
i am not using ssl certificate there what i feel is its not accepting http and only accepting https