Gumroad is a great tool for selling directly to readers. To capitalize on the powerful Gumroad sales interface, you need to first set your book up for sale as a product within Gumroad. Click here to learn how to set up your book as a product in Gumroad.
Once your book is set up within Gumroad, you need to integrate your Booklaunch.io page with your Gumroad product. You need the Gumroad product ID pertaining to your book to make this work. You can set that ID on the product page within Gumroad.
Finally, you probably want to add Gumroad as a purchase option on your Booklaunch.io page. You'll want to use the full Gumroad link (e.g. https://gum.co/demo). Note: the "/demo" portion is your Gumroad product ID.