You can run a BB30 crankset on a frame with BSA.
raceface and Rotobikes make BB's for that:

http://www.raceface.com/components/bottom-brackets/cinch-bb/cinch-bsa30/

http://www.rotorbikeusa.com/products/bsa_bottom_bracket.html

EDIT: NEVER MIND, you said GXP not BSA. sorry.

So, if i did this right there will be a bunch of photos of my IP-057 build and some of it after i was done.
I have to say this is by far the best bike i have ever owned. its coming up on its 1st birthday. i should do something nice for it.

im using the same lefty on my ip-057, 2011/2012 100mm 29'er w/ XLR,  i picked it up on craigslist with a wheelset (stans ZRT arch rims / c-dale lefty hub front and dirt flea in the rear) for about $300 less than what this add is asking for. while i am a BIG lefty fan i think this price is a little high. remember, any lefty from 2005 up to 2012 you can remove the internals and upgrade them to the 2012 latest greatest. id ask the seller to come down a bit and if not is recommend you keep looking. dont give up hope though, Leftys are worth it.