Sort all even numbers in ascending order and then sort all odd numbers in Descending order Java
You already dug into the standard library a bit (which is good) and discovered the beautiful sort method. Using this method is a perfect start.

What you overlooked is, that there is an overloaded second version of sort which accepts a comparator, so that you can perform all kinds of custom sorting with a single call.

Thus, instead of splitting the list in even and odd parts, sorting them separately, reversing one part, gluing it all back together, you could simply have created a comparator which encapsulates the logic, passed this into the sort method, and be done.

(BTW: even if you take your approach, reversing a list should be done with Collections.reverse())

So: create a comparator, do it all in a single sort call, and I look forward to see your revamped solution here in a day or so. And while you are at it: "GFG" is not an acceptable class name: what does that class do? Give it a name worth remembering!