Ok I think I have an idea:
You can use a variable to keep track of whether the user is editing an existing item or creating a new record. You'll need to update the OnSelect properties to set the variable.
To navigate to the Edit Form screen when editing an existing item, set the OnSelect property to something like this:
Set(varIsNew, false); Navigate(EditFormScreen)
Update "EditFormScreen" to your actual screen name that you are navigating to. To navigate to the Edit Form screen when creating a new record:
Set(varIsNew, true); Navigate(EditFormScreen)
Then for your Item property of the form, try:
If(varIsNew, Defaults('YourSharePointList'), Gallery1.SelectedItem)
It will probably need to be tweaked a bit, I don't know enough about how you are creating a new item