"Equilateral" means "equal sides". It is not a synonym for "regular", which means "equal sides *and* equal angles". Now, it's true that an equilateral hexagon that you can inscribe in a circle must be regular, but the GMAT certainly would never expect a test taker to know that.
So if this were a GMAT question, it would tell you the hexagon is regular, not that it is "equilateral". If we connect the center G of a regular hexagon to each vertex (A, B, C, D, E and F) we divide the hexagon into six equilateral triangles. The edges GA, GB, etc, of each equilateral are each radii of the circle, so they are 8 units long, and since the triangles are equilateral, the edges of the hexagon must also be 8 units long.
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