The article itself. The source presented by them don't show the quoted page, and I couldn't find any other source for that in a quick Duck Duck Go search.
So, is there any truth to this story?
The sole source of the claim is from the intelligence officer himself, Richard Meinertzhagen, from his war "diaries." Any time you're dealing with a single first-hand account of something that sounds extraordinary it's worth being skeptical about whether or not it happened (or at least as to its efficacy). I also put "diaries" in quotations because much of his published work was added in later. You can see an overview of his (un)reliability as a source here: https://books.google.com/books?id=FcBMAgAAQBAJ&pg=PA104&dq=intelligence+officer+opium+cigarettes+gaza&hl=en&sa=X&ved=0ahUKEwjj9vTh8o7XAhVs04MKHYuXC5EQ6AEILzAB#v=onepage&q=intelligence%20officer%20opium%20cigarettes%20gaza&f=false
But the story is amusing enough that he's been cited frequently elsewhere, for example:
If nothing else, it does capture the difficulties the British faced in the stalemate at Gaza. In the Second Battle of Gaza the more orthodox British wonder weapons of the day, tanks and gas, failed to break through the Turkish lines. It was a massive cavalry charge at Beersheba that finally did the trick and sealed the British victory at the Third Battle of Gaza.