Can you clarify how GAC 200 is best used in hard-edged painting?
The best use of GAC 200, a hard low-tack acrylic medium, for hard edge painting is to blend it with the acrylic paints and mediums. By adding this product, you reduce the elastic nature of a fresh acrylic film and this means hard edges will be cleaner.

Even a 15% addition of GAC 200 into paint will begin to lower the elasticity. It is also useful to blend with another medium, like Polymer Medium or GAC 100, and paint this over the inseam of a tape line. The medium will seek any crevices. When the intended paint films are applied over the dried medium, it cannot seep under the tape and when the tape is removed the line will be smooth.

Ratios will depend on how much trouble you are currently having with achieving hard edge lines. It's something that will require some experimentation, but in the end, you should notice an improved line.