At the moment, GSX sends a passenger bus and stairs, and in this case passengers running around the apron, I get the feeling it's too soon for multi jetway and gsx, heavy gates will need stairs for the time being
Is this working as intended then, gsx sends a bus and no jetwys detected, at least with one jetway, pax boarded through the jetway, I've even re-installed everything ie gsx and EGLL, plus you never know if the ini jetway will freeze sim
just to add, this happened to me twice when testing at Heathrow, no way out other than to restart sim, luckily it worked when arriving from Washington, but testing today and i found it happened