GBs occupied

go to your cc web site by opening cc desktop app>clicking cloud icon>cc web empty all 8 folders (4 in your files, 2 in shared with you, 1 in shared with you, 1 in deleted).

note: if you delete from your sync folder, your local files will be deleted, too. this is the only folder that affects your locally stored files.

click the lightroom link at bottom left of your cc web page and delete files from there. lastly, recheck your deleted folder and delete whatever is there.

if you still cannot account for your 20gb, contact adobe support per @Manan Joshi 

Did you check if you have some files present in trash that are still occupying space?

To contact support follow the instructions given below
1. Go to https://helpx.adobe.com/contact.html?rghtup=autoOpen
2. Wait a bit on the page that opens up, it should automatically open a chat popup. If that does not happen then look for a call out/chat icon at the bottom right, click over it and the chat popup should open
3. In order to talk to a human type "AGENT" in the chat box
4. If you are not able to get to the chat window, then disable your adblockers, firewall, enable cookies/scripts on your browser. If it still fails then change your browser/machine/network.

P.S. Be mindful of answering anyone who sends you a private message. See the following for more information about scammers
https://tinyurl.com/y7cfrr7y
-Manan