I dug around on this because I was hoping to listen to it myself, and the situation with 'Gatekeeper' is a bit mixed depending on which one you're looking for. If you mean the sci-fi novel by Christopher L. Eger, which is a sort of alternate history Cold War thriller, it looks like it's primarily available as an ebook on major platforms like Amazon Kindle. An audiobook version doesn't seem to be readily listed on Audible or similar services, at least not under that exact title and author pairing. Sometimes with lesser-known titles or indie publications, the audiobook comes later if the book gains enough traction.
On the other hand, if you're searching for something else with the same title, it gets trickier. There's a young adult fantasy novel from the 90s by Katherine Applegate called 'Everworld: Gateway to the Gods' that was sometimes just called 'Gateway' or confused with 'Gatekeeper,' but that series has ebook re-releases. The best approach is to search the title alongside the author's full name on a site like Google Play Books, Apple Books, or Kobo, as they often have a wider range of ebook formats than Kindle alone. For audiobooks, checking Libby through your local library can sometimes surface productions that aren't as prominently marketed on commercial sites. It's one of those titles where your exact result hinges on getting the author's name exactly right.