Allow (restricted) editing of source XML to add deployable properties that can't be fetched via Metadata API
It is impossible with OOTB Gearset to deploy an autonumber field with a starting number and optional populate existing rows. Why? Because the SFDC Metadata API as of V42 doesn't allow fetching of these two properties of the CustomField object:
Once in source, they'll deploy just fine to the target org. Note that changesets support this feature.
The workaround, which is to deploy to target branch and then edit target branch is sort-of-OK but it would be nice to modify this in the source. Of course, if you Refreshed Comparison, your edits would be lost.
I would limit this edit feature to only known places where the metadata API won't fetch a property but will deploy that property.