His exact course of action is up for debate and clearly impossible to prove either way, but I don't remember Keown getting sent off in a big game for a little foot flick after the ref had already booked him (and that is a key detail).
Rightly or wrongly he usually was clever enough to avoid leaving the field all together and that's the least we should be expecting from them. If they can't do that, somebody at the club needs to have words.
If Gabriel was determined to walk he should have just knocked the prick clean out and given us a happy memory like the one McN just posted above.