What if black holes evaporate because gravitational waves?

No. A symmetrical mass such as a single black hole will not emit gravitational radiation. What will emit gravitational waves would be two black holes ( or any dense binary system such as two neutron stars) rapidly rotating around each other. The gravitational wave will carry off energy from the system.

It doesn't mean matter is stuck. Gravity also becomes infinite past the event horizon. So how would you explain "gravitational waves" expanding and contracting. Though there are theories like hawking radiation which my explain how a black hole evaporates but even Stephen hawking recently has been forced to say the theory is incorrect.