Captain reported receiving a GPWS alert when aircraft entered an embedded thunderstorm with heavy rain; turbulence; hail and downdrafts that resulted in aircraft damage.

Narrative

We were scheduled to do a flight from ZZZ-ZZZ1. We checked the weather and radar and saw the thunderstorms were to the west of ZZZ1. The thunderstorm cells were scattered and there were some large gaps that we could navigate around to get into the airport. While enroute; we had the aircraft weather radar on. When we got closer to the airport; we asked for weather deviations and certain waypoints to keep us out of the thunderstorms. In route to ZZZZZ waypoint; the large gap we had been watching; started to close up. We asked for deviations to the south to get around the thunderstorms. While in IMC and using the Wx radar to deviate; we encountered moderate turbulence. We deviated even further to the south. We encountered heavy rain; and turned another 30 degrees to the right. We then encountered hail at 18;000 ft. in our descent(embedded TRW) and what we believe was a downdraft from the storm. Then we got a GPWS warning of Terrain; Terrain; Pull Up in our descent to 16;000 ft. The First Officer was flying and had to bring the thrust levers to idle; to keep the aircraft from over speeding. We leveled off at 16;000 ft. and broke out of the clouds; south of the storms. Most of the terrain in that area is 10;000 ft. We then saw the hail damage on the leading edge of the wings. All systems on the aircraft were normal and the aircraft was flying normal. I had no idea what other damage had been done to the aircraft; so I wanted to land as soon as possible. We then proceeded directly to the airport and landed. And on post flight; discovered other hail damage to the aircraft.