"Greg's Shirt" is an item that first appears in "Lion 3: Straight to Video" in Lion's Dimension with Rose's stuff. Rose acquired the T-shirt as a free item for "buying" Greg's album, Space Train to the Cosmos (which was free as well). Currently it still resides in Lion's Dimension.
Appearance
It appears to be a solid black shirt with a red "Mr." on it and a blue "Universe" beneath it has both red and blue streaks going across. This particular shirt is in the size XXL.