I find that difficult. In order to highlight just how offensive (offensive is an inadequate word) using an expression like that is, then yes.
Presuming that the perpetrator was stupidly unthinking rather than deliberately offensive, I would like to find some way of saying to him, with a message...
Yes, it was clearly a joke.
But as N's B says, it is the inclusion of the words "GAS THE JEWS" which makes it absolutely unfunny, stupid and sick. I doubt if Tony Bloom is laughing at it. Even if the trigger words had been "Sieg Heil", it might not have been quite so bad, or at least not quite...