git config does not set vimdiff layout properly, git 2.39

Question

I'm trying to configure vimdiff as a git merge tool, so my global git config contains the following lines:

[merge]
    tool = gvimdiff
    guitool = gvimdiff
[mergetool]
    prompt = true
[mergetool "gvimdiff"]
    layout = "LOCAL,MERGED,REMOTE"

but I am keep in having the default layout. A similar problem has reported here but it won't apply to my case because of the following reasons:

1. I am using gvimdiff
2. I am using git version 2.39.2 (Apple Git-143)

How should I solve it? Running on MacOsX Ventura.