Can't it be built off the corner post with no gate on it? Has that one been sealed, maybe? I seem to recall having the same situation recently and I was able to build both of them.
Not sure I understand the report, but it seems reasonable that you can't build a 2-tiled gate to cover a 3-tile gap.
"The psychological trials of dwellers in the last times will be equal to the physical trials of the martyrs. In order to face these trials we must be living in a different world."
IIRC the issue is that on 'build gate' the server, if there is more than one possibility to build one when there is more than one cornerpost in vincity, selects one of them in a deterministic way, in case it's the wrong one there dosn't seem to be a way to select the other possibility from that particular cornerpost.
Possible solution could be to have the server sort the possible gates building places by distance to the player and pick the closest one to the character (instead of the first).