For usage documentation please visit https://doc.babylonjs.com/extensions and choose "materials library".
To install using npm :
npm install --save @babylonjs/core @babylonjs/materials
Afterwards it can be imported to your project using:
import { GridMaterial } from '@babylonjs/materials/Grid';
And used as usual:
// Some awesome code
let gridMaterial = new GridMaterial("gridMaterial", scene);
// Some more awesome code
For more information you can have a look at our ES6 dedicated documentation.