Back at Gridbox Editor, drag and drop Gumroad component from Components Panel -> Integrations
Change the button text if you want to. (Click on the button and click on </> icon from the blue highlighted toolbar -> change the text and save)
Click on settings icon from highlighted toolbar wrapped around the selected button or open the properties panel from the left sidebar.
Enter the Product URL of the Gumroad product you'd like to associate with this button (for example,
https://gumroad.com/x/xxxxxx), which can be found under product share tab in Gumroad.
Under share click on the Embed Tab, copy the the url inside