The heartstone would be severed. Each side of the gateway is a different location in spacetime. So if it suddenly closed part of the stick would be at the starting location and the other half at the destination. Technically this must hold true otherwise heartstone wouldnt be able to cross through a gateway. If it can be half way then it is already severed until being moved fully through the gate. Rand has brought heartstone through gateways so it would be a yes to the sever.