It's about progressives. While you'll probably be understood just fine in most cases, "Hvad ser du" is technically asking what you see right now, this very instant. "Hvad er du i gang med at se" is progressive, meaning it's asking you what you are in the process of watching, not just right this second but maybe for the past hour and a while.
Danish doesn't have a special verb form to express this progressive nature, like english does with its -ing form, instead we use forms like "i gang med at".
They're pretty equal. I can't think of an example where there's a difference.
- Vi er I gang med vores lektier.
- Vi er i gang med at læse en bog.
- Vi er i færd med at læse en bog.
- Vi er i færd med en bog/lektier/madlavning.
So maybe only i gang med can be used with a noun, but both with an infinitive clause.