The PCs recently went through four sessions in a gate destination - the "Forest Gate."
The adventure itself was based on two sources. The map - heavily modified - and the basic skeleton of many encounters came from Rob Kuntz's "Garden of the Plantmaster," done up in a weird not-AD&D writeup. I liked the basic concept, but I really didn't love a lot of the details and some of the execution. The storms, the effect of cumulative damage on the hostility of the place, the idea of cleansing water, and a dragon's blood warping a guardian tree all came from "The Black Heart of Ulom," an adventure in Dungeon Magazine #11. That adventure was a bit too linear, but the idea behind it was excellent. Finally, the "demonic dragon" I wanted to have die over the sapling and warp it was simply that - a demonic dragon. Then Monsters 2 fell into my lap with the Bayanganaga (and the Treetrold, which had better developed and designed stats than I had for my own tree-men.) So naturally it became a bayanganaga, so the next time I use one of them the PCs have some idea what they're facing.
Also, Garden of the Plantmaster had weird warped elves. Elves? Nah, vegepygmies. My game is about vegepygmies. It always has been. They were perfect, and I built a lot around including them.
The minis were, of course, important. I'd had Mossbeard painted for over 15 years, and I got his lieutenants off of Miniatures Giant once I saw them post-Bones Kickstarter.
So all in all, the Forest Gate owed a lot to those two adventures. But neither really suited me and neither can really lay claim to being the primary source. I needed both, or I'd have run neither . . . and I had a lot to add on my own, too. That's GMing 101 though, isn't it?