THe "Forbidden" message I have gotten when there was an issue with the license file. Once I copied it back out to the Bin directory, the message went away.
There was nothing that I saw in the DotNetGallery control that would handle the Permission to only people that are logged in.
I had to do this on the page that contains the DotNetGallery Control.
On Page_Load you can use this function to control the ablity to restrict upload to members only
If CBool(Session("IsMember")) Then
gallery1.ShowUpload = True
gallery1.ShowEditDescription = True
gallery1.ShowBatchBuild = True
Else
gallery1.ShowUpload = False
gallery1.ShowEditDescription = False
gallery1.ShowBatchBuild = False
End If
Cheers