ArenaNet appears to have placed the upcoming Guild Wars 2 Gem Store items, most likely tied to Halloween, into the game files with the latest patch. A data miner on Reddit managed to dig these files up and give everyone a preview.
Considering that these items were data-mined, there is no official confirmation that they will in fact be the Halloween ones but the data miner, u/that_shaman, stated they are expected to land on 16 October 2018, which will be around the time Halloween festivities will kick off.
The first cosmetic item on the list is Scaled Dragon Wings Glider, shown on the picture above but considering it's only available in the preview window, you might have a hard time seeing the cosmetic to full extent. Therefore, it might be best if you check the preview in-game yourself, by pasting the code [&AgGXWwEAAA==] in chat and right clicking it for preview.
Same update will bring what some fans might find to be the best backpack or wings skin up to date. It is named Scaled Dragon Wings Backpack and is pretty much the wings of the glider above when they fold. You can check the glimpse of it on the screenshot bellow or in-game with the code [&AgGgWwEAAA==].
There will be a new chair apparently, named Night Night Watch Stool as well as Black Lion Expedition Contract. The latter will be a rather useful home instance item that will likely enable players to send out expeditions to retrieve "exotic resources" from Dry Top, Silverwastes, Bloodstone Fen, Ember Bay, Bitterfrost Frontier, Lake Doric, Draconis Mons and Siren's Landing. You can preview the description in-game with [&AgGTWwEAAA==] code.
Unfortunately, there is no preview function for chairs so the Night Watch Stool can't be previewed at the moment. On the other hand, the on Reddit features art previews for the stool, glider and the expedition board. There is also one interesting tonic that also can't be previewed, but the name "Fake Joko Tonic" gives us a good hint what it will turn players into.