As seen in the example or in the tests and sample that come with gtkaml, there is a prefix (usually referred to as xmlns:g="http://gtkaml.org/0.2" or xmlns:gtkaml="http://gtkaml.org/0.2", or xmlns:class) that contains attributes that are not part of the Gtk+ Vala bindings.
Here are their specifications, assuming the prefix is declared with xmlns:gtkaml.
The name of the class you're creating. May contain fully qualified name.
Allowed only at the root tag level.
The name of the namespace your class resides in.
Allowed only at the root tag level.
The comma-separated list of interfaces that the class implements.
Note that the inherited class is the root tag name.
Specifies a name under which the current tag becomes a public class member.
Mutually exclusive with gtkaml:private.
Specifies a name under which the current tag becomes a private class member.
Mutually exclusive with class:public.
Specifies that the current widget doesn't get constructed, but it configures a an existing widget. Also, it gets added to the parent container, unless it's standalone too.
The value is the identifier/expression under which the existing widget is reachable.
The expression is cast to the tag's name (e.g. <VBox gtkaml:existing="dialog.vbox1" spacing="1" /> becomes (dialog.vbox as VBox).spacing = 1)
Allowed only on non-root tags.
Specifies that the current widget doesn't get added to the parent container.
The only valid value is "true".
Allowed only on non-root tags.
Specifies code to be executed just before adding this widget to the parent container.
The code may use target as the name of the current widget, just like with a signal
Specifies code to be executed just after the instance is created.
The code may use target as the name of the current widget, just like with a signal