The trick is to find something else that your grey likes to destroy. I had a similar problem with Alfie when he discovered the shelves in the living room. He threw everything off the shelves then started chewing them/pulling them apart. I discovered that he loved to destroy cardboard boxes so I found some that fit perfectly on the shelves. Now, when I let him out, I clear all my things off the shelves and leave a few cardboard boxes and toys up there for him. He will happily sit and shred the boxes for hours and leaves the shelf well alone.
The chewing is a natural behaviour- you just need to find a way to redirect it to a more desirable object if you can. (e.g boxes, old bits of paper, telephone directories... anything he can chew and shred)
Depending on your cabinet- is it possible to tile the tops of them? One of my colleagues also has a grey who loves sitting on the top of the cabinets- so they put tiles on top. Can't be chewed and are also wipe clean! He also fills the space with various toys and boxes to keep his grey busy/distracted.