Usually the gate can be braced at 4 blocks in radius (like if you built 4 wood planks perpendicularly to the gate), not even mentioning the 8 block radius for Minas Tirith's main gate, which is 3 blocks thick.
(Tested on several gates)
but this one gate, second level in Minas Tirith, has only 2 blocks radius.
https://imgur.com/a/wEjF5Mf
Another gate bug, I found on Edhellond.
Basically someone braced all of the gate on 5th level (the battle was happening at 4th level flag [not market, the other one]) while on other gates on other maps, you can NOT put blocks there before the flag is captured.
https://imgur.com/a/V434sQY