Maybe their skill can allow gate guardian to use the "incoming attack zero" effect 3 times? I mean gate guardian is so OP in the anime yet it is utter garbage in the actual game. Really looking forward to find out how they are going to salvage this useless card.
They don't need a skill. The cardpool for NPCs is far more open than the player's cardpool. There's nothing stopping them giving the Paradox Brothers cards like Soul Charge if they really want to. I just hope they have a strategy aside from Summon Gate Guardian pieces and the occasional Jirai. That isn't going to be the most interesting Duel ever, lol.
It will definitely be a skill. They're not going to go through the whole shebang of giving us the Brothers as a playable character and have this whole animation to go with it if Gate Guardian is virtually unplayable. Bakura has a Skill that allows him to automatically win the game, so it's not convoluted to think that there can be a skill that bypasses traditional summoning conditions.
In addition to that, Gate Guardian is not very popular as a strategy (a laughingstock, more like), nor are the Brothers very spectacular as dimensional or fan-favourite characters. They have to make a skill that directly supports the playing of Gate Guardian to not only give the Paradox Brothers actual skills of substance, but to make Gate Guardian plausibly playable.
It could very well be a skill that bestows an immediate advantage at the start of the game, like how Bakura and Pegasus did. Also, like Bakura, they might also have supplemental non-skill bonuses that can help them right off the bat. It will definitely not be starting Gate Guardian on the field, because that's just bonkers. Keep in mind, this doesn't mean this skill will necessarily be good.