The gravitational wave equations form a parametric resonance system during oscillatory reheating after inflation, when cast in terms of the electric and magnetic parts of the Weyl tensor. This is in direct analogy with preheating. For chaotic inflation with a quadratic potential, this analogy is exact. The resulting amplification of the gravitational wave spectrum begins in the broad resonance regime for chaotic inflation but quickly moves towards narrow resonance. This increases the amplitude and breaks the scale invariance of the tensor spectrum generated during inflation, an effect which may be detectable. The parametric amplification is absent, however, in 1st-order phase transition models and "warm" inflationary models with continuous entropy production.