Hi @Dpalmer ,
Do you want to know how to set a gallery's Default property?
Firstly, let me explain the meaning of a gallery's Default property.
It means that the gallery's default selected item.
If you do not use any formula in its Default property, it will be the first item in the gallery by default.
For example:
If you set an edit form's Item to: Gallery1.Selected
If you do not have any formula in Gallery1's Default property, the edit form will display the first item by default. (before you select any item in the gallery)
If you use a formula in Gallery1's Default property , the edit form will display the item that you set in the Default property by default.(before you select any item in the gallery)

Secondly, what formula you should use in a gallery's Default property?
As I've explained, the Default property is used to set a gallery's default selected item.
So you need to fill in a formula that represents a record inside the gallery.
Now, I use the case that you mentioned as an example:
1)set the navigate button's OnSelect in gallery2:
Set(var,Gallery2.Selected.ID)
2)Set gallery1's Default:
LookUp(data source,ID=var)
//replace with the data source in gallery1
//filter the same item that you select in gallery2
Then the form that its Item is gallery1.Selected will display the item that you select in gallery2 by default.
Best regards,