Hi,

Before we address your concern, we must check certain aspects first.

Notice that it says "Design Failed (No Optimization)". This is because the the design exceeded the typical flux density. The design has a peak flux density of 7230 Gauss, where typically the maximum value is 3800 Gauss. This must be addressed first by going to Active Design -> Start Optimization or by following the suggested fixes in the next column.

As for the core gap, you can not change it directly. As an alternative, you may adjust the Inductance and the PI Expert Tool will recalculate the core gap. Also, you may follow the suggested fixes on the next column.