Currently gawk is in the system sets, but this will possibly change in the future. One could port vim.eclass, but I don't feel comfortable rewritting the awk part of apply_vim_patches into posix awk. And anyhow system without gawk are mostly minimalistic systems, which will most likely use busybox's vi instead of vim and hence don't use this eclass.
+ 10 Mar 2013; Christoph Junghans <ottxor@gentoo.org> vim.eclass: + add sys-apps/gawk dep