Build the urls inside of the service.

app/helpers/issues_helper.rb

@@ -16,32 +16,19 @@ module IssuesHelper
   def url_for_project_issues(project = @project)
     return '' if project.nil?
 
-    if project.default_issues_tracker?
-      project_issues_path(project)
-    else
-      project.external_issue_tracker.project_url
-    end
+    project.issues_tracker.project_url
   end
 
   def url_for_new_issue(project = @project)
     return '' if project.nil?
 
-    if project.default_issues_tracker?
-      url = new_project_issue_path project_id: project
-    else
-      project.external_issue_tracker.new_issue_url
-    end
+    project.issues_tracker.new_issue_url
   end
 
   def url_for_issue(issue_iid, project = @project)
     return '' if project.nil?
 
-    if project.default_issues_tracker?
-      url = project_issue_url project_id: project, id: issue_iid
-    else
-      url = project.external_issue_tracker.issues_url
-      url.gsub(':id', issue_iid.to_s)
-    end
+    project.issues_tracker.issue_url(issue_iid)
   end
 
   def title_for_issue(issue_iid, project = @project)

app/models/project_services/gitlab_issue_tracker_service.rb

 class GitlabIssueTrackerService < IssueTrackerService
-
+  include Rails.application.routes.url_helpers
   prop_accessor :title, :description, :project_url, :issues_url, :new_issue_url
 
 
@@ -10,4 +10,16 @@ class GitlabIssueTrackerService < IssueTrackerService
   def to_param
     'gitlab'
   end
+
+  def project_url
+    project_issues_path(project)
+  end
+
+  def new_issue_url
+    new_project_issue_path project_id: project
+  end
+
+  def issue_url(iid)
+    "#{Gitlab.config.gitlab.url}#{project_issue_path project_id: project, id: iid}"
+  end
 end

app/models/project_services/issue_tracker_service.rb

@@ -22,6 +22,10 @@ class IssueTrackerService < Service
     # implement inside child
   end
 
+  def issue_url(iid)
+    self.issues_url.gsub(':id', iid.to_s)
+  end
+
   def fields
     [
       { type: 'text', name: 'description', placeholder: description },

spec/helpers/gitlab_markdown_helper_spec.rb

@@ -23,6 +23,7 @@ describe GitlabMarkdownHelper do
     @project = project
     @ref = 'markdown'
     @repository = project.repository
+    @request.host = Gitlab.config.gitlab.host
   end
 
   describe "#gfm" do