Is GIS a word in Scrabble?

Yes, gis is a valid Scrabble word.

Is GI a word in English?

A GI is a soldier in the United States armed forces, especially the army.

Is IQ allowed in Scrabble?

No. To make it clear right now, IQ is not a valid word in Scrabble. This is according to the official Scrabble dictionary, even though IQ is a legitimate word in the dictionary. That’s because, generally speaking, you cannot use abbreviations in Scrabble.