In some parts of the World (e.g. Line 14 on the Parisian metro) there are gates and doors. Gates on the platform, and doors on the train. Mind you, some people will consider the platform gates as doors (like a lift/elevator).
I understand that both are possible but not in the same context.
The first proposed idea could be used when a mother warns her child: "Don't get closer! The gate is not safe".
The other idea may fit when a person is worried about the gate and say: "The gate is not safe; our property is vulnerable".
I wrote "The gateway is not secure" can "O portão" be used for "Gateway" ?