Board Thread:General Discussion/@comment-25550913-20150307051012/@comment-50.21.131.196-20150314053524

For me it appeared when Gitchi Manitoxu was around lv12 (all others boss was already minimum lv20).

For my friend, he beated all boss around lv20 too except one at lv8 (he doesn't get much time that day) and the Gate did not appear yet for him.

So it seems The Heaven's Gate must certainly start appearing after you beat each boss at least a number of time, maybe 10 or 15 time ... I guess.