I believe The Gardener is The Traveler. IT fought The Traveler and won, because The Traveler did not shrug its shoulders and make itself alone. Meaning it did not sacrifice those it was protecting. It did not let go of the burden/responsibility of protecting humanity. IT(the Darkness) doesn't care for its minions, protecting them is not a burden IT carries. And that is why IT always wins. Rasputin learned a lesson in ruthlessness. We built him to be stronger than us, and learn. He learned that being alone, without the responsibility of protecting anyone, will allow him to be ruthless enough to defeat IT.