I had this problem with an alt today, and i have the solution. I had chosen Goblin Engineering. I completed the quest but didnt like what Goblin had to offer so I dropped Engineering and retrained in it. Once I got back up to the appropriate level, I couldnt seem to train in Gnomish; the Goblins wanted me to renew my membership with them even though I was no longer a Goblin Engineer. After speaking to 3 different GM's who all gave the wrong advice, I asked a guild mate and he told me! There is a book on a table inside of a house north of steamwheedle port in Tanaris. This book lets you train in either Goblin or Gnomish. I hope this helps. If you cant read the book, try abandoning the quest and talking to the goblin in Ratchet again.
PS - Gnomish Death Ray FTW!