Just like it says on the title I was wondering if I could use Cisco GSR12000 router as BRAS to manage clients via PPPoE
C12000 GSR is a backbone router that can have edge services depending on type of linecards installed.
However, BRAS features should be not supported on it.
Cisco 10000 is the main platform for this job.
C7200 or ASR 1000 can be used too.
Hope to help
How many users you realistically need?
Alos consider that if you use regular ethernet/Ip encapsulation, you will simpligy the infrstructure very much, gain on performances and manageability. It's not like PPPoE is mandatory for broadband.