Not really.

We use the rules of the partner carrier for stopover purposes, but make exceptions to those rules for connecting city purposes.

"Nearest Gateway City" is really meant to describe a situation where the partner doesn't service the city you're flying to/from. If you are wanting to fly JNU-TPA or ALW-TPA, AA doesn't service JNU or ALW so AS would provide service to the nearest AA city (generally speaking - SEA).

But the reality is that AS lets you use their flights even if the partner does fly there. And as long as you're traveling in the same general direction, will allow connections (but not necessarily stopovers) beyond the published routing.