I want to talk in a chatroom with some user to discuss a topic. But that user has less than 20 reputation. Is there any way I can gift him 20 of my reputation points?
No. While bounties technically allow transferral of reputation from one account to another, they are not meant to be used as gifts, and any bounty that is found to be used that way will be refunded.
The user will have to earn the necessary reputation points on their own. It's not that hard - 20 points are just 2 upvotes.