In attempts to aid on time performance, BA have a procedure at LHR called ‘ASO’ (Adjusted Schedule Opportunity). The TRM/Dispatcher in conjunction with the BA Airport Control Centre will identify if certain criteria are met, including reduced load factor and then apply an ETD change of up to 10 mins *note this is not an STD change*. This is usually done around -60/70 to the STD. It will show in the BA app.
But the key point remains, as others have noted, that the gate close time remains -20 to the STD, not the updated/adjusted ETD.