How does Gearset build a comparison?
Behind the scenes, Gearset runs a listMetadata call to gather details about the components in your org. From this, it builds the package.xml files and uses them to retrieve the metadata via the Metadata API.
What factors go into deciding if a component is part of a managed package?
Gearset uses some of the information given in this listMetadata call in order to decide whether or not to include a component in the comparison grid.
Specifically, Gearset uses the manageableState of the component. If the component shows as installed then it will only be included if that namespace is included in the Include managed packages section in the metadata filter.
Therefore, if this manageableState is not installed then it will always be included in the comparison, as long as it aligns with any other filtering.
How can I test this?
You are also able to run these listMetadata calls easily and for free using an external tool called Workbench.
To do this first login to the org on Workbench, and then go to info > Metadata Types & Components.
From the list, find the metadata type that you are wanting to test and expand the folder. In this example I've used layouts.
Within the list you will be able to see the manageableState of the component. In this example we can see two layouts, one is the layout that came with the package, and the other is a custom layout that I had created on the managed package. I would expect the custom layout to appear in the comparison regardless of whether Include managed packages was set to "All" or "None".
How to exclude any custom components built on top of a managed package from being in the comparison?
As in the example above, if you’ve built custom components on top of managed packages (e.g. custom layouts on a managed object), they’ll appear in your comparison because they are considered unmanaged.. Since the manageableState is unmanaged it is included within the comparison.
To exclude these, you can use Regex filtering in your metadata filter settings.
As shown in the screenshot, this filter is set to exclude any components with the FSL__ prefix, ensuring they’re not included in the metadata comparison.
More details on Regex filtering can be found in this article: Using regex in custom metadata filters.