I added and commited a branch locally (in a Docker container mounted as volume and locally in Windows).
The file contained tabs which had a width of 2 spaces. (they had two spaces in neovim and in Visual Studio Code)
Then I pushed that Branch onto gitlab.
And in gitlab the tabs I added to that file had a width of 8 spaces.
I tried using the autocrlf option in .gitconfig but it did not seem to have an effect.
I did the add and commit inside the Docker container.
But the git push I had to do in gitbash because of authentication issues.
Since I still see two spaces in my local file and in the Docker container it must be the git push from gitbash that changed the tabwidth.
joanis answer was correct and led me to the solution.
I solved the issue by adding the set expandtab to my .vimrc which converts tabs into the number of spaces you set with set tabstop and set shiftwidth.
So I can do in vim and it produces actually two spaces instead of a real tab.
If I don't use set expandtab the tab is only 2 spaces wide, but it still is a tab and behaves differently in gitlab.