It is as simple as this, Breed Gummy Dragon + Gummy Dragon and you will probably a get Legendary Dragon! I´ve tried to times and both tries were succeseful, i would say it´s 100 % sure that you will get a Legendary Dragon by breeding these two dragons. Good Luck!
I received a legendary dragon by breeding my Gummy that was at least higher then a level 10 and my armadillo that was a level 10. I'm not saying you'll get it on the first try buts its worth the chance. My friend wanted a legendary and I breed their gummy + cool fire and it worked first time as well. Keep trying and good luck!