I'm not really aware of any sources of data that go down to that granularity.

I suppose you could write a script to try to parse it out from the heathrow website over time and collect the data.

But I guess my question would be - to what end?

Past gate usage at LHR doesn't reflect future ones. I'm a regular on certain routes and have found myself going from A, B and C, being bussed and not on the same flights on the same days.