Gulildmaster? I've beaten that storyline all the way through and never got that;... It is Theives' Guild no?
Yes. After Brynjolf declares you to be the new GM, you still aren't actually. You have to do five successful "help the guild" quests in each of Riften, Windhelm, Whiterun, Solitude AND Markarth (there don't seem to be any - that I found - for Winterhold, Dawnstar, Falkreach or Morthal). The problem is that they don't tell you where the job is until after you accept it, so then you can either do MANY extra jobs in places you have already triggered, or turn the "bad" jobs down after accepting them - which counts as a failure.
Eventually you will notice four extra merchants in the Flagon, as Tonilia steadily gains more money to buy your loot. Finally Brynjolf will have a ceremony making you the official GM, and you'll be directed to Tonilia to collect your new GM armor and amulet. Here are the wiki links