Gibraltar is easier to defend, but Calais is easier to supply. Hugely fortified, you would probably need control of the Channel to take it.
It could be held, it's just unlikely; and it would probably require history to take some different turns.
It is going to make things interesting for the Spanish Armada. It is where they anchored and the English sent in the fireships. Then again Bolougne would probably do equally as well.