SetProperty(control.propertyName, propertyValue)
This is the same functionality as the Power Apps SetProperty function.
When working with a nested gallery, use Index() within the SetProperty function.
SetProperty(TextInput.Text, "Say Something")
SetProperty(Dropdown1.Selected, {Value:"2"})
SetProperty(ComboBox1.SelectedItems, Table({Value:"1"},{Value:"2"}))
SetProperty(Index(Gallery1.AllItems, 1).TextInput1.Text, "Change the text input")
Select(Index(Index(Gallery1.AllItems, 1).Gallery2.AllItems, 1).TextInput1.Text, "Change the text input")