Gaffer uses the term "Render Passes" and the RenderPassEditor UI for render layers. Murray posted an example file in this thread:
https://groups.google.com/g/gaffer-dev/c/iQ9z-gUmlX4/m/Jpy-3ZudFAAJIf you play around with that, and the RendePassEditor UI, hopefully that will get you started.
As for slap comps, you can load your images in Gaffer with the ImageReader node, and do basic compositing.
Templates are somewhat up to you how you want to structure things. The simplest version of a template would be just having a file that contains a basic setup for your pipeline, and then a script that copies that file and opens it when you want to start working on a new shot. The next step might be put the standardized functionality for your pipeline inside one or more Reference's, which can be published more centrally ( perhaps at a sequence level ). That means the functionality inside the references can be maintained centrally, and you can do fixes to the references which will then be loaded next time you open any shot.
A more complex version of templating would be the ImageEngine pipeline, where the sequence template is locked down, and shot artists work inside special boxes that are the only places you can edit. When you load the sequence template in a shot context, asset management automatically loads the content of the shot boxes for that shot. This is pretty complicated, but does allow for some very powerful sequence workflows.
-Daniel