I don't know any "unofficial" ways to deal with this that you have not heard. I have wondered if a group "intervention" might work, but I have never seen it tried in a workplace. I have tried talking bluntly to some difficult people. The results have varied. I think the times when I got some good results, I had kept their point of view in mind. Be prepared to have more than one such conversation.
Getting anyone to change behavior may be very difficult if the behavior has been allowed for a long time. If unofficial ways don't work, then you will need the help of someone above the gaslighter. (But that is an "official" way to do something, which was not your question.)
So below this point is a compromise of some "unofficial" ways to get "official" help.
You might discuss your concerns with managers or directors who are not involved, but are in nearby or related departments which share upper management. Pick one that has been in the company long enough to know upper managers. They can tell you which person up the chain will probably listen.
If there is a group that goes out for food and drinks after work occasionally, go with them. You may get more open advice away from the company's office. You may find that they start the conversation by asking you about the gaslighter.
Do all of this respectfully. If you come across as just bashing the gaslighter and your own boss, you may hurt yourself. Try to remember that you may want a job reference in the future.
When you find the right person to approach for "official" help, keep the company's interests in mind while making your case.