You can do this programmatically by setting the gallery's SelectedItemCategoryFilter property to a filter from the gallery's ItemCategoryFilters collection. For example, after invoking the following line, the gallery's content will be filtered using the filter with index 3.

[C#]
gallery1.SelectedItemCategoryFilter = gallery1.ItemCategoryFilters[3];

[VB.NET]
gallery1.SelectedItemCategoryFilter = gallery1.ItemCategoryFilters(3)