No. Grand Slams are ITF events, with different categories (singles, doubles, mixed...) with different trophies and prize money for each...
The Olympics transcend tennis, the gold medal you get from winning singles isn't any different or less meaningful than the gold medal you get from winning doubles.
I do understand your point about this context, but are the Olympics important enough strictly as a tennis event to be part of the Hexagon? Not sure they are, after all they offer less ranking points than a Masters 1000.
Of course he can always say, 'I have a gold medal'.
But the gold was in doubles, and I believe, for similar reasons as to why Davis Cup is not included, that a doubles gold medal shouldn't go towards achieving such a feat as this. It's a measure of ability in the singles game, not the doubles game.
Anyway, let's agree to disagree. I understand your point, and absolutely recognise the symbolism of any Olympic gold. Anyway, let's not get too worked up about something this trivial.