Lol It seems at every airport you have to request “taxi to parking” to get taxi to a gate. Heathrow is one of those. Many other airports have gate labelled as Ramp GA SMALL etc despite those parkings being attached to a terminal with jetways. No GA aircraft I know of uses jetways.
If you get sent somewhere dumb like that by the madmen atc in this sim, just ignore them and taxi into a sensible gate of your choosing. Another bug that needs fixing.