Goo is a mysterious substance obtained from the bosom of ghosts during Elvin Gadd's experiments. Elvin found out about this strange gooey substance and labelled it on the Periodic Table as well; Goo is 00 on the Periodic Table, thanks to Elvin Gadd. With this viscous gooey substance Elvin Gadd molded and formed the Goo into Luigi 's shape, and while successfully forming Luigi , E.Gadd put on a video of Luigi busting ghosts. As the Goo is Luigi's clone--it then start to mock the video of Luigi's performance and even develop its own Poltergust 3000 model, thus Gooigi is born. Nowadays Gooigi is used by Luigi now to help eradicate ghosts, although looking extremely similar to Luigi, it's beneficial actually.
(Gooigi) Luigi's Mansion
(Gooigi) Luigi's Mansion: Dark Moon