And that is why I am not a bard. Or an evil overlord.
If I was one of those, who were to hide the gate, I would place it in an adamantine sphere, encase that in a lead sphere, then pile a mountain of mostly non-commercial rock. There would be some galena and a small amount of other ores to make it look natural, but not enough to make it a viable mining spot for anything. Then I would have a castle built on top of it and start a city around it. None of the people involved with the castle and city should know that there is something important in the mountain.
The mountain would be created with repeated usage of Wall of Stone.
As a 20th level caster, I could get almost 5,9 m
3 per casting. With minimum Intelligence of 15 to cast Wall of stone, I would get 4 of each type or relevant slot, 20 slot total. Filling all possible with Wall of Stone, I could cast it 20 times/day, to build almost 117,9 m
3 of stone. If I had any helpers, it could go faster.
I would use galena as the most common ore, so that anyone, who knows there is a gate in the mountain, but not where would be confused if they tried to magicaly search for lead.
It's also possible that the Gates need some kind of maintenance. I mean, even the gods themselves couldn't really keep the rifts shut, and the Gates are basically just putting duct tape over a spot weld.
Possible, though doubtful. The one in the pyramind was in a lead column, while the one in Azure City was in a gem. Both would be pretty hard to get to to maintain. Especialy the one, which was a gem as a part of a throne.