i am not convinced this is some HAL conspiracy.

with short haul it's very difficult to announce gates until 50-60 minutes because gates do change a lot during the day. until the aircraft is getting to the confirmed gate it's not really a good idea to be sending folks to B and C since it can change, and it is a longish walk from B and C back to A.

with long haul i think there may be a case for a bit more notice - but then gates for long haul do get announced a bit sooner. also you will often see "Will depart from C gates" against a flight on the HAL screens maybe a few hours before a flight - not to indulge the conspiracy theories but that hardly seems the action of an organisation desperately keeping passengers in T5A.