[2024.2] Help with Geckolib Custom Model Entity not rendering item in hand

I figured out that the normal [Living Entity] can add layers which allows me to make an custom model hold an item with ease.

However the Geckolib lacks this feature; and my model does not hold said item, so I tried and failed to edit the code to make an attempt at doing the layer trick.

I tried to research into this with past forums for around an hour and found honestly nothing. Just people saying to use Geckolib, then not explaining how, or saying to do the "program the item in the Model". The latter of which will be annoying, and unwanted, as I wanted the model to swap between items you hand them. 

So I am asking if there is any knowledge of this.