In the story of the Odyssey there was a Great Horse called the Trojan Horse that was brought into the city Troy. Inside this great horse were armored men called the Greeks. They stayed in that horse until nightfall. Once it hit They emerged out of the Horse and destroyed the city of Troy also ending the war. Is this part of Greek Mythology Myth or Truth?

Truth. Anything not involving mystical creatures in greek mythology has truth behind it. It was a gift that they hid in.

I might even say some of their mysticism and mythology may have had some truth behind it as well.

Agreed, there is often a lot of truth in stories, some times it may be hidden between the lines. It is hard to say, how large and how many troops where hidden or how they came in with it, since many facts get confused in the story telling.

But even in the history of the US, the myth is often confused with truth

Is this part of Greek Mythology Myth or Truth?

It is unknown for certain whether the city of Troy ever truly existed, or whether Homer's accounts as told in the Iliad and The Odyssey are based in fact or just pure mythology. Some archaeologists believe they have found where Troy was situated, but there is no historical evidence of any specific conflict with the Spartans. There is no supporting historical evidence for the story of the Trojan Horse.