It depends on how the fanfiction is written. If the focus is on Ash's growth and development as a trainer, he will likely win. He has the spirit of never giving up and is always learning. But Giovanni has his own strengths. He has access to powerful pokemon and a lot of resources. So, if the story is more about Giovanni's grand scheme and his intelligence, he could be the victor.
It's really hard to say. Ash has his experience from all his travels and his strong bond with his pokemon. But Giovanni is a very cunning and powerful trainer. If Ash manages to outwit Giovanni's devious plans, he could win. However, if Giovanni manages to use his resources and underhanded tactics effectively, he might come out on top.