this is the patch for redmine to display custom_subject in the gantt chart.

redmine_custom_subject.patch

diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index 62c6fbb..2467a66 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -74,7 +74,12 @@ module ApplicationHelper
     if options[:subject] == false
       title = issue.subject.truncate(60)
     else
-      subject = issue.subject
+      if options[:use_custom_subject] 
+        subject = issue.custom_field_values.detect{|cf| cf.custom_field.name == "custom_subject"}.try(:value)
+      else
+        subject = issue.subject
+      end
+
       if truncate_length = options[:truncate]
         subject = subject.truncate(truncate_length)
       end
diff --git a/app/models/issue_query.rb b/app/models/issue_query.rb
index 5490585..b920890 100644
--- a/app/models/issue_query.rb
+++ b/app/models/issue_query.rb
@@ -89,6 +89,17 @@ class IssueQuery < Query
     end
   end
 
+  def use_custom_subject
+    r = options[:use_custom_subject]
+    r == '1'
+  end
+
+  def use_custom_subject=(arg)
+    puts :use_custom_subject=
+    puts arg
+    options[:use_custom_subject] = (arg == '1' ? '1' : nil)
+  end
+
   def is_private?
     visibility == VISIBILITY_PRIVATE
   end
@@ -119,6 +130,7 @@ class IssueQuery < Query
     super
     self.draw_relations = params[:draw_relations] || (params[:query] && params[:query][:draw_relations])
     self.draw_progress_line = params[:draw_progress_line] || (params[:query] && params[:query][:draw_progress_line])
+    self.use_custom_subject = params[:use_custom_subject] || (params[:query] && params[:query][:use_custom_subject])
     self
   end
 
diff --git a/app/views/gantts/show.html.erb b/app/views/gantts/show.html.erb
index 83fcd89..e84ee44 100644
--- a/app/views/gantts/show.html.erb
+++ b/app/views/gantts/show.html.erb
@@ -49,6 +49,15 @@
             </label>
           </fieldset>
         </td>
+        <td>
+          <fieldset>
+            <legend><%= "use custom subject" %></legend>
+            <label for="use_custom_subject">
+              <%= check_box 'query', 'use_custom_subject', :id => 'use_custom_subject' %>
+              <%= "use custom subject" %>
+            </label>
+          </fieldset>
+        </td>
       </tr>
     </table>
   </div>
@@ -105,7 +114,9 @@
   @gantt.render(:top => headers_height + 8,
                 :zoom => zoom,
                 :g_width => g_width,
-                :subject_width => subject_width)
+                :subject_width => subject_width,
+                :use_custom_subject => @query.use_custom_subject
+               )
   g_height = [(20 * (@gantt.number_of_rows + 6)) + 150, 206].max
   t_height = g_height + headers_height
 %>
diff --git a/lib/redmine/helpers/gantt.rb b/lib/redmine/helpers/gantt.rb
index f8990d4..bfe21ae 100644
--- a/lib/redmine/helpers/gantt.rb
+++ b/lib/redmine/helpers/gantt.rb
@@ -380,7 +380,7 @@ module Redmine
                              :size => 10,
                              :title => assigned_string).to_s.html_safe
           end
-          s << view.link_to_issue(issue).html_safe
+          s << view.link_to_issue(issue, options).html_safe
           subject = view.content_tag(:span, s, :class => css_classes).html_safe
           html_subject(options, subject, :css => "issue-subject",
                        :title => issue.subject, :id => "issue-#{issue.id}") + "\n"

Usage

1. Create a custom field named "custom_subject" in the administration screen.
2. Relate a custom field to your project
3. Create a ticket that has a custom subject
4. Display the gantt screen
5. Toggle "use custom subject" checkbox on in the option pain
6. Click a apply button