Gavelkind
Gavelkind has one large advantage: Your demesne can be 30% larger. If you could normally have a demesne 8 holdings large for example, you'd with gavelkind be able to hold 10 holdings without any problems. This can make expansion easier as you don't have to immediately give up gained titles to your vassals, you can instead hold onto them until you have a suitable vassal to give them to.
It also gives 30 extra opinion with every eligible heir except the oldest, 10 opinion with the rest of your dynasty, and 5 with every other vassal.
Gavelkind does however have a rather major drawback. As your titles will be distributed between your children you will almost always end up with fewer titles upon succession, and if you hold multiple kingdoms your realm will be split. It also gives you -10 opinion with your oldest eligible heir.
One workaround for this is to ensure that there is only ever one eligible heir, and this can be done in a few different ways.
The first one, and the simplest one, is making any extraneous heirs bishops as bishops cannot inherit. This is however only possible if you have any bishoprics to grant.
Second, you can assassinate or banish any extraneous heirs. The first however has the drawback of risking the Kinslayer trait which reduces your relations with your entire dynasty and reduces your diplomacy stat, while the second induces tyranny and thus makes every vassal you have angry.
Third, you can ensure that no more than one heir is ever born, either by getting the celibate trait, or by killing your wife. The first method is far from surefire, while killing your wive risks you getting Kinslayer.
A major drawback of the 2nd and 3rd methods are that if your primary heir somehow dies before you do, you're now left with no heirs. Unless you then manage to get another heir in time or change succession laws it would be game over.
First time in over a week I actually got some time to waste.
