I tend to just try and reverse image search. In this case, it was also a bit of luck with the snapshot of the gif that I used.

To start with, I reverse image searched a snapshot of the gif. I used Yandex’s search image search engine for this, but it’s best to try multiple. With luck, it returned similar images to the snapshot, so I visited the sites. Initially, I checked the related images section on the first web page to see if the gif popped up, but it didn’t. So I copied the title “FFM Pussy to pussy switching teen threesome fuck” and used Google search. I visited the second link. Checking the related section of this web page, a thumbnail of the gif was present, so I visited the web page that the thumbnail linked to. Below the thumbnail was a link to the video that the gif originated from.