I see the Heathrow Express benefit is now listed on the GGL Benefits page:
Upgrade to Business First on Heathrow Express*
As a Gold Guest List Member, our partners at Heathrow Express invite you to enjoy a free upgrade to their Business First class. Save time with priority carriages, ensuring a quick arrival and departure from the platform. When onboard take advantage of increased privacy and leg room, and complimentary magazines and newspapers. Or stay connected with complimentary wifi, at-seat power and spacious worktables.
Purchase an Express class Heathrow Express ticket from
heathrowexpress.com/avios, the Heathrow Express app, in stations or on board the train. Take a seat in the Business First carriage and simply show your valid Gold Guest List card or app along with your ticket.
*Business First upgrade is bound by the
Heathrow Express Terms and Conditions.