"Greg's Shirt" is an item that first appears in "Lion 3: Straight to Video" in Lion's Dimension with Rose's Stuff. It appears to belong to him and it could've been used by Greg when he was younger. Currently it resides still in Lion's Dimension and it's former owner is Rose Quartz.
Apperance
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.