I don’t ever use gvim myself (meant to look into it for years but never have). But you can find my config file in an old post here. I do think it makes it a bit difficult for new users that a decent default vimrc isn’t included in the haiku package.
There are probably some things in my vimrc you don’t want (and these days I have even more stuff), and I think that the move to the new forums has mangled the spellchecking options in my post. The most important though are “set nocompatible” and “syntax on”.