Know how after the ambush at the cabin you have to choose one of two ways to go, the Bella Sisters or El Gigante? I almost always take on the El Gigante. This time Ashley ended up dying twice (because she ducks and stays still if you point your gun in her general direction, I had to do this to hit the El Gigante with the rock) and I was tired of this, so I ran to the door on the other side of the canyon. I had her wait by the door so she'd be out of the way, then lured the El Gigante away. I noticed the key in the wreckage of the cabins, grabbed it, and surprisingly enough the door actually opened. In the past I had always stuck to fighting it in the area you start in, then combing the area for whatever I could find.
Monster Hunter Tri is one of the best Capcom games of this generation. Of course, that's not saying much.
The wooden gates have chains on them clearly indicating you're supposed to be running away or that there's a locked door like you noted and that Leon mentions that 'there might be a key back there' or somewhere along those lines not to mention you're placed into a bottle necked area where it would be a little difficult to fight El Gigante unless you knew what you were doing not to mention you have Ashley to take care of and regardless of if the cabin is destroyed or not as well as if you even kill El Gigante or not the key will always be there.
Edit: almost forgot, the boulder that Ashley points out and are placed at specific points? They're obviously meant to incapacitate El Gigante enough for you to get away/take care of the chains on the wooden gates.
There is a Ghost in my attic and he says: "It's very dark up here...." ....Oh right....and he also says: "Boo."