I want to buy a downpipe for my 02 gtp, but i have lots of questions before i actually get it.
Should i get a 2.5" or 3"?
How hard is it to install, is their any directions anywhere i could follow?
How much would it cost to have done somewhere?
What would i need to install it?
What kind should i get? Will it fit everything else thats stock?
Answers to these questions, and maybe anything else i should know is greatly appreciated

http://www.grandprix.net/ubb/ultimatebb.php?ubb=get_topic;f=1;t=009463#000000

That only helps with a few of my questions. Maybe one of them

1. GTP=3"
2. You can buy a bolt in downpipe from ZZP, 4 bolts off and 4 back on and you're done
3. You would have to go somewhere and ask to find out
4. Sockets, ramps, and PB Blaster, maybe new gaskets
5. Yes

The larger the downpipe, the less resistance, the less the torque. The GTP's supercharger eliminates the issue of torque loss, yet gains from the free-flowing exhaust by reducing KR.

Short answer:
Boosted: 3"
N/A: 2.5"

Sockets, ramps, maybe new gaskets
does the zzp downpipe come with these?

Here's a write up: downpipe install You can order front & rear gaskets from ZZP, or from your local auto parts store: Front Gasket: 31605 (Walker)
Rear Gasket: 31532 (Walker)

No you kind of need to provide your own tools

You'd be better off having someone else do it because unless you have a cutting torch all the pb blaster in the world is not going to do it believe me i know. It doesn't cost much $100-150 tops.

It really isn't that easy unless you have no rust and have plenty of tools, including an impact gun and tons of extensions.

this all sounds like a u bend delete isa better, because its wayyyyy cheaper, easier, and still reduces kr and give 5 horsepower, where as the downpipe is 15?

If you wanna go further with your mods, you would be better off getting the DP, which includes the u-bend delete. I would get the ZZP DP: http://www.zzperformance.com/grand_prix/products1.php?id=551&catid=110

Most people who get u-bend deletes regret not just doing the whole DP. Maybe not right away, but if you plan on continuing modding, you will regret it.

I dont plan on doing too much more, atleast not for a good amount of time. I just purchased a brand new u bend delete w 02 sensor and gaskets for 35 bucks . Good deal

not really that good of a deal... you have to pay someone to install it too