Problem solved. For anyone who may be stuck with a similar issue, the required configuration was:
gitlab_rails['env'] = { 'http_proxy' => "http://proxy.myorg.ch:3128", 'https_proxy' => "http://proxy.myorg.ch:3128", 'http.nonProxyHosts' => "...", 'GIT_SSL_NO_VERIFY' => "true" } -- You received this message because you are subscribed to the Google Groups "GitLab" group. To unsubscribe from this group and stop receiving emails from it, send an email to gitlabhq+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/gitlabhq/5d93e511-e462-4c12-ba50-2fa37b8b5649%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.