I used to do the same kind of work. The Unsemantic Grid is great because it’s responsive, but it uses percentages so you won’t get things to be pixel for pixel as in the design.
However, depending on the design it shouldn’t matter too much and you should be able to get quite close.
You can use pixels in a grid system, but you’ll run into issues when it comes to responsive design.
Let me know if you need more info 🙂