There is a lot of skill/art in getting a flat surface with this type thing.
For best results, you're expected to sand thoroughly in between each of several coats put on with a small nap roller, or use a pressure sprayer in a spray booth(which takes experience).

Goo is just a mixture of white paint that originated when people saw that the audiophiles (and their wallets) started crossing over into videophilia, bringing their competitive appreciation of niche products/techniques to color temperature, gain and reflectivity. The high cost of high-end screens, the subjectivity of "white," and the words 'specially developed,' made it popular.
Go with any of the dozens of mixes of paint that have been popularized on the AVS DIY screen boards. You can try half a dozen of 'em and pick whichever you like better, putting them on 4x8 tackboard, for the same price as a tub of Goo.