There are several archetypes and I don't use all of the project types;
so patches for the archetypes are more than welcome!
The maven eclipse plugin doesn't know about the eclipse gae.
You would need to send those guys a patch or correctly setup the configuration for it.
Since you started out with an archetype, you could argue the archetype should contain the needed code; send us a patch, and we will include it in all our archetypes.
Just do a clean before gae:run.