Sometimes when you make a comparison between between two orgs, you will not have all Letterhead  items show up in the comparison.  The reason for this is often your letterhead is using an image that is not in a public folder.  Salesforce will not allow Gearset (or any other platform) access to the metadata for any items that use private documents.

For example, I have three Letterhead  items in my Salesforce org (Setup -> Search for "Letterhead" in the quick find box):

If we run a comparison on the Letterhead metadata in Gearset, only the NoLogo and HasPublicImage Letterheads will show up.

To get the LogoLetterhead  to show up in our comparison, you need to check which images are used in the Letterhead: Select the item -> "Edit Letterhead".

Once you have found which images you use, go to the Documents page in Salesforce ("+" -> "Documents").  If you do not have a public folder, create one:

Make sure you select "This folder is accessible by all users":

Now find the item you want to move ("+" -> "Documents") and find the document (either look in the folders or use the "Find Document" search).  Select the Item and "Edit Properties".  Change the folder to the public folder:

Now when you do a comparison in Gearset, this Letterhead  will now show up:​
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To get a successful deployment, any Data  items you reference, will need to be already on the target org, in the same folder structure you have on your source org.  
If not, you will have an error such as In field: logo - no Document named APublicFolder/Logo.png.

​Note: Due to a quirk in Salesforce we are able to list all three items if we look at the items to include and exclude in the filter, even if we cannot make a comparison with them.