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 isn't 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.
While the deployment succeeds for this deletion of the Workflow Actions, as Salesforce does not throw any errors in the validation, when you check the target org the change is not applied and the Workflow Action still exists.
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.