Grid now uses Shopify's Product recommendations feature to suggest similar products to customers, based off of the product they are viewing and other relevant data.
Activating product recommendations
To activate this, navigate to the theme editor, then open a product page in the right-side preview. In the left-side settings, open Product recommendations and enable Show dynamic recommendations.
In here you can also customize the copy for the Heading.
For more information on the Product recommendations, check out Shopify's article:
Upgrade from related products
Releases of Grid before v4.5.0 would use the Related products feature. Related products will pull from the collection associated with the product, ordered by whatever order that collection has been set to, and ignoring itself in the list of products. If the product belongs to multiple collections, it will pull from the first one (alphabetically).
To upgrade from Related products to Product recommendations, update to the most recent release of Grid: