On the deployment summary page, items with No difference may appear in your deployment package, even though they were not selected for deployment. This is because sometimes Gearset will need to deploy additional metadata components to make a deployment work.
For example, custom fields aren't actually their own metadata type. They're a subcomponent of a custom object. To deploy a custom field that was selected, Gearset needs to redeploy the parent custom object.
Another example is when deploying field-level security / permissions changes to profiles - the parent profile will also be included in the deployment.
No difference indicates that Gearset is redeploying metadata that Gearset has downloaded from the target environment of the deployment.
If you want to understand the changes that Gearset is about to make to the target of the deployment in more detail, you can download the deployment package from the deployment summary page and inspect its contents.
Example scenario
On below pre-deployment summary page we're seeing that Gearset has automatically included Property Custom object ('No difference' item) into the deployment package, while I've only selected Street_Address Custom field ('New' component) to be deployed.
Gearset automatically included Property Custom object in order to make sure that validation of the deployment package succeeds:
While there are no actual changes made to the Property Custom object in the target environment.