Regularly, Salesforce releases new versions of the API for preview, during this period there are sandboxes that are on a new API version (e.g. v51 or Spring '21), when the rest of the production orgs are on another API version (e.g. v50 or Winter '21).
This document explains how Gearset handles the version changes.
Once Salesforce releases the new API for preview, Gearset releases support for the new API (e.g. v51) as soon as possible. At this stage, Gearset does not make the new version the default API, therefore automation jobs continue to use the previous API version and nothing changes.
People wanting to test the new v51 can set it explicitly in their comparisons or automation jobs.
Once Salesforce releases the new API for general release, Gearset will then make the new version (e.g. v51) the default and at this point all automated jobs will start retrieving with the new version of the API.