Unfortunatly 그게는 is incorrect. '그게' is the abbreviation of '그것이', so you cant say '그것이는'. You could say: 그것은 (formal, and correct grammar) 그것이 (formal, and correct grammar) 그건 (short version of 그 것은) 그거는 (another widely used abbreviation, even though it's not shorter haha) 그게 - The ones that aren't formal and correct grammar, are coloquial, and most people use them. And as Hyung Nam said, '맛있어요' is the polite version And '맛있습니다' is more formal.

It's delicious. "그게는" is wrong expression. The correct expression is below 그것은 맛있어요. or 그것은 맛있습니다. (more formal expression) Enjoy your learning!!

You don't usually put "그게는" when speaking, but it is right! 맛있습니다 is even more formal.