Can I put 20" rims/tires on a 2004 Pontiac Grand Prix GT and not cause damage to the fender wells or car?

You should measure your wheel wells for height and width. At one time years ago I bought some oversize snow tires for one of my vehicles. To mount them I had to let the air out and re-inflate them after they were mounted on the vehicle. It was a close fit. You may have to do this with the rear tires. I do not know what you can do for the front tires. I would find out more on the clearances. You will have to watch out about your turn radius on the front. You need to make sure when you turn, the tires will not hit anything. I would ask the dealer about the problems that can happen. He should know if they will fit and what problems can occur.

Ive seen 20" rims in a grand prix, I dont know if any modifications were made but I do know they looked awful.

Yes you can but make sure you have a very low profile tire!

Zinik rims are very nice i was thinkin bout getting 19" for my acura tl type-s