My Kitchenaid unit (refrigerator on the top, freezer on the bottom) started building up a lot of frost kind of suddenly. I inspected the freezer door gaskets, and they seem to be sealing normally. What would be the next steps?
It could be that the door was left slightly ajar or that something was caught in the gasket when the drawer was closed, then someone else noticed later and fixed that problem. Then you come along and notice the frost, but no cause.
(The frost on the top of the drawer looks suspicious to me. That might be an indication of where an item was stuck in the gasket.)
Now that you have confirmed the gasket is seated properly, let it be for 24-48 hours. The automatic defrost should clear it away.
A common cause for this problem is failure of the fan unit in the back of the freezer compartment that circulates the air inside the refrigerator/freezer. You would want to determine if this fan is still working properly.
Note that the fan is typically hidden behind a plastic panel and not directly visible. Exact details regarding how to access and remove the fan will vary according to the make and model of your refrigerator.