My heir only received all of one territories, one I conquered recently. The title I created went to another heir, and the rest went to a third. What the hell?
Did you not have any "title loss" warnings?
When you become a duke, you will no longer lose territories other than the ones outside your duchy. Same goes with the kingdom.
To clarify what he means, he means that when you hold the duchy you can never lose territory in that duchy (unless someone who's going to leave your duchy happens to have some of his land in there). In fact, as soon as you have a duchy, I'm pretty sure you can never lose any county-level vassals or holdings except to other, already independent countries. Same with kingdoms and duchies.
However, if you form or get two duchies or kingdoms, or you just plain happen to be a count, and you don't have some higher rank that you have
only one of, you will lose all De Jure vassals and land to whoever Gavelkind set to inherit the land, as your sibblings will be inheriting titles of similar ranks to you. And dukes cannot be vassals to dukes. Same with kings. This is why it may be a good idea to avoid getting multiple duchies or kingdoms on Gavelkind unless you're going to quickly try and become a higher rank or change the succession law.