What's the difference between Pinot Gris and Pinot Grigio?

Identical grape, different winemaking intent. Grigio = lighter, crisper, drier (Italian style). Gris = riper, rounder, more textural (French/Alsatian style). Australian labels use the name that matches the style.

Usually dry to just off-dry. Richer Gris styles can carry a touch of sweetness for texture, but nothing like a dessert wine.

Is Pinot Gris related to Pinot Noir?

Yes — it's a colour mutation of Pinot Noir. The grapes are actually greyish-pink ('gris' is French for grey), which is why some examples show a faint blush tint.