Stew might have more info, but from what I've heard from several friends who work there is that new hires start in GIVSP regardless of previous types. At about the 2 year point you would get the opportunity to upgrade to GV, then later on the 450/550.
A lot of guys pass on the GV, because it just means 11 hours flying instead of 7-8 hours--and both jets go to all the same places. Pay does not change, and you will end up coming back to the GIVSP when it's time to upgrade to captain.