Simple Grid is described as 'CSS grid for your website' and is an app in the Development category. There are more than 50 alternatives to Simple Grid for a variety of platforms, including Online / Web-based, Self-Hosted solutions, Mac, Windows and Linux. The best alternative is Bootstrap, which is both free and Open Source. Other great apps like Simple Grid are Bulma (Free, Open Source), Materialize (Free, Open Source), UIkit (Free, Open Source) and Semantic UI (Free, Open Source).
Structure is a declarative, Flexbox based grid framework. Structure defines layout using non-standard attributes on elements. (If you have used frameworks like AngularJS, you have already gotten some exposure to declarative syntax and non-standard attributes.)