GSS was created b/c our company's main product, the Grid (still in stealth mode), is doing some radical things with design of newsfeeds, we need layout to be completely decoupled from the DOM. A fairly developed Float / Flexbox-based layout becomes as much of a CSS affair as an HTML one; changing layouts requires a whole new set of div wrappers. CSS was designed to separate presentation from content, and that ideal has unforeseen manifold benefit when actually realized.
Cassowary powered constraints allow for layouts not possible with a CSS unable to resolve cyclic dependencies in layout arithmetic. Of course, you could easily shoot yourself in the foot & define fragile, ambiguous and/or impossible layouts. But, w/ great power comes great... I always prefer expressive freedom over easy-to-comprehend limitations.
GSS is very new - much is needed like better debugging, implementing floats, etc.. Constraint-based layouts isn't a silver bullet, it takes practice & thinking more like a programmer. Fortunately, Apple Auto Layout demonstrates the viability of all this craziness.
Furthermore, there's quite a few basic constraints that CSS implies that you'd need to reimplement in GSS (if we can call it that) all over the place to get a comparable layout engine, namely normal flow, margins, wrapping, justifying...
All in all, I'm not holding my breath for this to become a huge success.