Someone forked my private repository and released a product with it

Hi there!

[Legal story]
For years I’ve worked on a project with a friend of mine, me doing all the coding and he the art.
When we decided to work together with a company (without a contract), a few months later the company broke off the working relationship, and went on developing the game themselved.
When that happened, I explicitly said that I did not give them any permission to use my (standard, unlicenced) copyrighted code, and they said they would rewrite it all.

Now, with the company releasing the project, a meta file made me suspicious. It was the exact same format I used to make, with some serialization (it was a save file) having classnames and variable names exactly the same.
I just looked into my GitLab repository after a few months (I’ve had given up the development basically) I noticed a ‘‘private fork’’ of my repository.

They claim the product has all rewritten code, but (with their consent) I looked into the code, and it has 70-100% the same stuff in the classes I made.

What can I do now?
Please help! (It’s a russian company, that changes things i’ve heard…)

I hope people will see this and respond, as I’m desperate now worrying for my copyrighted code.

Thanks in advance.