Yes, greengasradio server allows cheating. No particular software is required, just a basic knowledge of console commands that I don't want to write here (you can try yourself at home if you start a server configured to allow cheating).
Most players don't notice the advice at the beginning of the game, so they don't understand how it is possible that some opponents are invulnerable or get all weapons at once or always know where you hide, as they could see through walls. And even when they understand that someone is cheating, they don't understand that they can do it too, nor they know how to do it.
I think greengasradio has made a
wrong choice allowing cheating, a choice that could possibly hurt SG game in its entirety.
Please, if you play in greengasradio, tell exactly to other players the truth about the rules in this server, so every player is informed and can decide if stay there or go elsewhere.
Apart from this, if you want to stay away from "real" cheaters choose a server well administered and controlled by administrators (should I suggest one or two?...
). But, in my experience, cheating is not so common in SG, and the most part of cheating accusations simply comes from new players to experienced ones that play fair.