Изменить настройки файла /etc/gitlab/gitlab.rb на:

external_url 'https://example.ru'
gitlab_rails['gitlab_ssh_host'] = 'example.ru'
gitlab_rails['gitlab_host'] = 'example.ru'
nginx['redirect_http_to_https'] = false
nginx['listen_port'] = 80
nginx['listen_https'] = false
nginx['proxy_set_headers'] = {
  "Host" => "$http_host_with_default",
  "X-Forwarded-For" => "$proxy_add_x_forwarded_for",
  "X-Forwarded-Proto" => "https",
  "X-Forwarded-Ssl" => "on"
}
nginx['real_ip_trusted_addresses'] = ['xxx.xxx.xxx.xxx/32']
nginx['real_ip_header'] = 'X-Real-IP'
nginx['real_ip_recursive'] = 'on'
letsencrypt['enable'] = false