There's nothing untoward happening here unless Gasnier keeps getting paid after he retires, in which case it should still count under the cap.
What will be cheating, but unfortunately still legal, is if Gasnier's playing contract (counted under the salary cap) is replaced with a coaching contract (which is not counted under the cap). Kind of like how Darren Lockyer has been playing for the Broncos for peanuts because he'll be compensated with a fat coaching contract after he retires. It might be cheating, but it's not illegal.
And the guy who said players are allowed to play for less than their market value is correct. The only exception is that players aren't allowed to renegoiate the terms of existing contracts unless the new contract is also an extension. That's why the Storm players couldn't just take pay cuts.
Anyway, the NRL should still look into any instance of that happening because it's sus, and goes against the spirit of the salary cap.