It's the bogus claim that grace believers are gnostic...it's just because certain people don't understand about the new creation in Christ - created in righteousness and holiness and that the old man is dead....they can't figure out spiritual truths because they are operating from their own natural mind.
So, to them the new man in Christ is a mystery and to them it makes it look "gnostic" ..you will soon find out that anyone that doesn't agree with their views are gnostic whether they believe in the gospel of the grace of Christ or not......it's the nature of the beast...