Using the online catalog, search for the author you want to link via the template.
Find the author listing, such as: Harrison, James A. (James Albert) (1848-1911)
Edit the wikipedia page and add in the Gutenberg author template. Set the 'id' variable equal to the author's name in this format: Lastname,+Firstname or however the name appears in the Gutenberg website, usually lastname followed by comma and then the rest of the name. The 'id' sets the actual link to the Project Gutenberg database. The plus "+" are required; if spaces are used, the URL won't work. Although it's possible to use a bare template with no ID parameter since it will default to the article title, this is not recommended as articles on Wikipedia are continually renamed for disambiguation purposes and it's safer to lock in the desired ID.
You may also set the 'name' variable in the template to equal the author's name how you wish it to appear, such as: James A. Harrison or James Albert Harrison. The 'name' is for Wikipedia display purposes only.
Dates in the ID should be avoided unless needed to narrow the search results. This is because the dates in the Gutenberg database may be incorrect and will change in the future thus potentially breaking the URL. Nevertheless, it's better to add the dates if they are needed to narrow search results.