What is the best product to glue the floor down? With concrete? With plywood?

The recommended adhesive for this material is an acrylic based vinyl adhesive. You will need to make sure that your adhesive is appropriate for your subfloor. Better Life Technology sells and recommends two adhesives (for plywood or concrete). https://bltllc.com/gfloor-universal-flooring/accessories/. Many retailers or online resellers also offer a variety of adhesives – just be sure it is an acrylic based vinyl adhesive appropriate for the subfloor. You can also visit this blog for more information about G‑Floor installations. https://gfloor.com/blogs/news/everything-you-should-know-about-using-adhesive-to-install-g-floor