It is -5 and lower, although you’ll almost never encounter a player with exacly -5.0 security status. If you are unsure, then the default overview will show them as flashy with an orange skull. If you are using a custom overview, then you can set your own aesthetics for outlaws.
It apples to every system 1.0 all the way to 0.0
If your sec status is -10 then you will still take gate guns for attacking a non-valid target on a gate.