Not so sure about this last bit. I always assumed that Gardulla bought Shmi (and Anakin?) elsewhere and lost them to Watto while on Tatooine. Given that she was there with Jabba at the Boota Eve Classic suggests to me she comes and goes from Tatooine on a semi-regular basis. Possibly an ally or rival of Jabba's or another member of his particular clan.
Sorry, I phrased my statement misleadingly. Yes, I agree that Watto moved to Tatooine before he bought the Skywalkers. I was just working through the chain of causality backward -- the Skywalkers lived on Tatooine because that's where Watto lived, and I'm guessing Watto lived there because of the podraces, and the podraces were there because of Jabba.