When deploying this single Custom field, there are two scenarios to consider: (1) the Object for the Field is new to the target; or (2) it already exists.
The object is new to the target
If it's a new custom object (i.e. it only exists in the source org), Gearset will automatically select all custom fields ready for deployment. However, you are able to select which custom fields are included within the custom object
In this example, there is a new Custom object with a few fields, when it is selected for deployment, all components are also selected, one is not ready for deployment, so I open the components menu and deselect that field.
Within Salesforce, a Custom field must always belong to a Custom object. Therefore, the components are not able to be selected without also deploying the object.
The object exists in the target
If the Custom object exists in both the source and target, you'll have the option to choose which Custom fields you want to deploy.
A Custom object that exists in both source and target will look like this:
Note that Opportunity shows metadata for both source and target, so the custom fields can be selected for deployment.
To deploy a New Custom Field from the source to the target, you can select the field from the New items tab (as shown below). It will also be nested under the Custom object, if you prefer to find it that way.