Gearset has full support for spinning up and deploying to/from scratch orgs - you can read more about that process in the scratch orgs configuration walkthrough.
Unfortunately, due to Salesforce API limitations, any scratch orgs created by other tools such as the SFDX CLI tool cannot currently be accessed by Gearset, and so won't be shown in the list of scratch orgs.
We will continue to monitor this situation and hope to add support for scratch orgs not created by Gearset as and when the API allows us to.
Accessing scratch orgs created in Gearset, via the CLI
If you've created a scratch org in Gearset and now need to access that in another tool, such as the Salesforce CLI or Visual Studio Code, then you can do that by resetting the password for the scratch org. To find out how to do that, click here.