Hah, that's a blast from the past.
I remember seeing those ads, and then my elementary school teacher showing us how to make 'we have gak at home', basic non newtonian fluid, via mixing water and cornstarch in the right proportions, roughly 1 cup cornstarch to 1/2 cup water.
Apparently there is another fairly basic recipe that is closer to the very synthetic, actual gak, but is not non toxic, so you could try that with older kids, or I guess nostalgic, non suicidal adults: