That link is still active, you just have to wait about 10 seconds for the download to begin.
I "created" the slug generator because a Gap is literally "nothing". It is just a way to put space between clips in the timeline/storyline. A slug is transparent video and can be used to fill a transition's placeholder.
BTW, you can use FCPX to generate your own slugs, just use a Basic Title and delete the text (or replace it with a space character.) Then export. If you use ProRes 4444, the video will be transparent. If you use anything else, the video will be solid black. You could also use any generator with which you can set the opacity to 0.
Tom is probably right. There are many ways to create custom transitions and there are, no doubt, many that will affect all media from their "layer" and everything below, very much as Title effects do. In titles, the "Drop Zones" (technically, they're "placeholders") gather up everything that is below the title, including other connected clips into the placeholder media (items above the title's level in connected clips are not placed in the placeholder media.) It's one way the user can create their own "on the fly" effects in FCPX by gathering them under the effect of a title.
One sure way to get "secondaries" to "behave" with transitions it so group them into a Compound clip. Using compounds don't really add any extra "weight" to a project (all references and uses XML) and does not duplicate video clips. Compound clips can contain Luma blending modes (Stencil Luma or Silhouette Luma) that usually will cut holes through everything under an effect including media *under* the storyline. An example of this use is to "put video inside text" and black out everything else. If you put the Title and the Video to fill the text inside a compound clip, you can contain that consequence to just what's inside the compound, place the compound over the storyline and have your storyline video show will you have a different video fill the text of the title (not that I'd ever recommend that!)
There is a method to the way FCPX was put together which is not quite how a lot of us would have thought to do it. In the end, FCPX tends to be more flexible than we could have ever imagined. Just accept it and work faster 😉 What's that commercial? It's not how fast you mow, it's how well you mow fast? I'm sure there's an analogy in there somewhere...