I was told that going gf for at least 12 months allows damaged guts a chance to heal and therefore many people that have previously had problems with gluten can reintroduce it slowly with no ill effects.
Ds1 was gf for four months, then had to put him back on it for several months before his coeliac test. We noticed a big difference behaviourally between him gf and eating gluten.
His coeliac test was negative, but we were told he is most likely non-coeliac wheat intolerant.
Unfortuately, since putting him back on gluten for the test, he has become extremely resistant to eating the gf food. So we now limit the amount of wheat and gluten he eats, keeping him pretty much wheat/gf, but allowing him to eat wheat at birthday parties and other social occasions. Not ideal, but at least he'll do it without fighting at every meal.
We can always tell when he has eaten some wheat/gluten as there is a big change in his concentration, behaviour and there are also physical changes to his skin and bowels.
Ideally I would like to keep him strictly gf for at least 12 months, but I have to balance that with his happiness and anxiety levels and there's just too much else going on with him at the moment to force the point.