^Dawn might beat Nando this time but she will probably lose to either Kenny or Zoey. But who knows maybe we will never see Nando again.
I don't think they'll have her lose to Kenny against since she just lost a contest to him. So we've got Dawn, Nando, Kenny, Zoey, and possibly Jessie and Kotone...wow..
Dawn - She'll probably beat a few GF-only characters and Kenny but then lose to Zoey..
Nando - I think he'll be the older, more mature character that wins the GF. Like Robert and Solidad before him
Zoey - I think she'll make it further than Dawn or Kenny but lose to Nando..
Kenny - He'll probably lose to Dawn in the middle of the GF
Jessie - I really doubt she's going to make it at this point. Unless there's some kind of time skip between when Dawn wins her 5th ribbon and the start of the GF there's almost no chance. A time skip like the 3 month period Ash usually has to wait for the league after earning his 8th badge. Jessie could easily earn them off screen with a short time skip but even if she made it to the GF I doubt she'd make it far
Kotone - We don't even know if she's a coordinator yet, I'd rather her be a trainer/coordinator than a Pokethlon trainer though. And if she is a ccordinator she might not even be competing in the Sinnoh GF, she could just be visiting from Johto like May did.