I have been through this. I'll tell you now that it is not easy.
Read This link on everything.
The link above will actually give you a whole insight of the GB TCG.
Goodluck, its pretty tough.
After you defeat the 8 Masters, you will have to go to the Dome where you will battle Grand Masters and after that the Final Champion. Its a bit like the Elite 4 but this is TCG.
These are the types of the Grand Masters.
Save before battling each one.
Once you defeat the 4 Grand Masters and the Champion, you will receive a Dragonite, Aritcuno, Zapdos and Moltres.
I know this is similar to the other answer but I wanted to answer since I have played through this.