Is a grommet needed for romex into this metal junction box?
You don't use a grommet in this application but rather an appropriate clamp.  The correct type depends on what you are connecting here but the usual thing for non-metallic cable is something like this:



https://images.homedepot-static.com/productImages/7b668807-9db0-42c3-86b1-e9ec7481045c/svn/halex-conduit-fittings-20511-64_1000.jpg