GAT prices are the same as
Disneyland Travel and some others, when purchased online. The advantage is purchasing them beforehand, as opposed to getting them at the gate. You save a good little bundle.
ONE Magic Morning (1-hour early entry to Fantasyland, Buzz and Space Mt) is available with EVERY 3-day or more parkhopper, no matter where you buy it. However, if you stay at one of the 3 Disney hotels (DLH, Paradise Pier, or Grand californian) you get to use all the MMs as a courtesy to their guests, although they don't advertise this on the website.
If you are staying offsite, the advantage to purchasing through GAT is that they are the ONLY authorized
travel agency that gives you TWO Magic Mornings with your parkhoppers. However, I just found out this is only through the end of this year. After that, they are back down to just one MM, like everybody else. However, park hoppers next spring are all on sale for kid's prices. So that's a good deal.
Mickey's ToonTown Morning Madness is a different thing entirely; you get into ToonTown one hour early (it usually opens an hour after the park does, so on M, W, F and Sat it's open at park opening for MTTMM). There's a little song and dance with the Fab 5 and you get an honorary citizen button, and then there's huge autograph lines the rest of the time. Cute, but not worth buying a package to get it. MTTMM only comes with purchase when you buy a hotel/ticket package through Disney, Costco or
AAA. Since you can usually save 100's of $ by booking separately, I don't recommend it if you're on a budget.
Hope that helps!
