You may find that when running a comparison in Gearset, no Apex classes appear for an org in the results view, even though that org contains some.
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The first thing to check is that Apex classes are included in your metadata comparison filter:

If Apex classes are included in the filter, then your Salesforce user for this org may not have permission to edit Apex classes.

For Gearset to be able to compare and deploy Apex classes, the Salesforce user you used to connect to the org must have the Author Apex permission enabled:

Once you've enabled this permission, Gearset will be able to compare and deploy Apex classes to this org as normal.