I recently had the heads off of my 1995 Grand Am for a bad manifold gasket. It has a 3.1 V6, most likely the same motor as your Grand Prix.
I spent about $400 in parts. I also replaced the valve seals and rocker arms as long as I had it apart. It took me a few weekends to do it in my garage. I figured this to be about 1 1/2 car payments so it didn't take long for me to come out ahead doing this repair.
Most auto supplies sell a head change gasket kit, sometimes called a valve grind kit. The kit will have every gasket you will need for the job.
If you decide to tackle this job, get a bunch of zip-lock bags in different sizes, some masking tape, and a sharpie. Label every wire you disconnect. For every component you remove, place all of the fasteners and hardware in a zip-lock bag and label the bag. Place any small components in a zip-lock bag and label the bag. The more organized you are, the better off you will be.
Take detailed pictures with a digital camera of the engine bay at various points while disassembing. I took a new set of pictures at least once a day. Take the pictures from more than one angle. It was helpful to look at these a couple of times to remind me how things went back together.
During most of the repair, the passenger side motor mount was removed. You will want an extra floor jack to place under the motor. Use a block of wood to protect the oil pan. You will be surprised how much you can move the motor around the engine bay with the motor mount removed.
While you have the heads removed, now would be a good time to change the spark plugs. Use some good platinum plugs and you will never need to change them again. Use anti sieze on the threads just in case.
If you go to the Auto Zone website, you can print all of the required steps to do the job.
VERY IMPORTANT! Don't get cheap and try to re-use the head bolts. These are torqued by a stretch value and can not be re-used.
