What about the gate to Moria? Could Gandalf just cast a 6th lvl spell to get in? Are you sure that spell is enough to get through the magic of True lock? You can portal through any solid material, but can you portal through let's say some kind of magic force field?
For the Gandalf example I think it's a false problem because I'm quite sure nothing like a portal spell exists in that setting, so Gandalf couldn't use it to bypass Moria's gate. Besides that, there's no evidence that a True Lock spell was used to make that specific magic door.
But if it existed and was made with True Lock, there are other factors to consider:
1 -
True lock says that
the door is locked not thet the room is unacessible like it was sealed by a magical field. You can't open the door nor break it, but you can tunnel under it, break the wall next to it or enter by the back door if there's one, so you could gate into the room.
2 - There are many gate spells, so it could vairy depending on which one is used, but I don't know any GM who would let a PG gate into a room he doesn't know well without knowing how big it is or the thickness of the wall. Some, and me among them, would rule that if you do such a thing you have a good percentage to end up materializing into solid stone and die. Just my opinion, of course.
As for the boots question, it really depends on how magic works in your world. In my games true magic objects cannot be dispelled, only destroyed. But I would say that the gate is a normal object with a permanent enchantment, while the boots are a true magic object.
