This problem analyzer warns you if you attempt to deploy an object on which a Workflow Action has been deleted from a Workflow Rule.
What triggers this problem analyzer?
It's not possible to remove a Workflow Action via the Salesforce metadata API, although new Workflow Actions can be deployed.
If you've deleted a Workflow Action, the change would not present itself in the target org, which is why Gearset alerts you with the error message:
Cannot remove WorkflowRuleActions.
If the deployment is part of a CI job, you would end up with changes detected on every run. (See our support article on how problem analyzers affect CI jobs.)
How do I resolve this problem?
You can manually remove the Workflow Action from within the Salesforce UI by selecting
Remove on the Edit page of the relevant Workflow Rule.