check previous discussions on ibm gbs (formerly bcs), should give you enough information on ibm's gbs.
1) it's an IT shop than consulting, it's all about hardware, applications, and integration
2) you will be nobody if you are in the delivery side of the business, only industry go-to-market (sales) prevail. all promotions go to sales (and don't be surprised if you find them not knowing what they are selling)
3) engagements are mainly, if not all, IT related "transformation". As long as hardware, applications, and system integration are sold in a bundle
4) performance management process is a joke. you are only as good (or bad) as what your "manager" said you are. period. so always have good relationship with those who have a say on your performance.