We have been here for 5 days now and have tried several different strategies to ride Gringott's. It has been easy to access Diagon Alley and the Hogwarts Express, but all our attempts to ride Gringott's were frustrated.
Yesterday we bit the bullet, got up at 5AM, and walked from Cabana Bay since the buses did not start till 6AM. We arrived at 5:45AM to find a short line. We were able to get in the first group that was allowed into Diagon Alley, but not without some hustle and a little running. When we arrived at Gringott's, the posted wait was 20 minutes, and we were able to ride without any delays. The ride is fantistic. Upon our exit the wait time was 240 minutes.
Another advantage to this strategy is that it allowed us to immediately leave Diagon Alley and be first in line to Despicable Me, Rip Ride Rocket, and Transformers before the lines became unbearable.
This plan is the only one to work for us. Hope this helps.