|Can a Path Alias refer to items on other Gateways?
No, even though they can reference them in the Gateway component of the XPath, Path Aliases can only be used to access items on the same gateway.
Path Aliases can only access data in the local gateway. Sometimes, like other XPaths, they can contain an explicit reference to a gateway name but this is either because the original creator of the configuration used a unique path via drag-and-drop or cut-and-paste or, less often, a Rule is in an include file and a Target or Path Alias is intended to only match Data Items on a specific gateway.
This also applies to other places that you can use XPaths in the configuration.
If you need to use data from another gateway then you should look at the Gateway Sharing feature. More information is available here: Gateway Reference Guide - 27 - Gateway Sharing.