Here's an example of some .vimrc commands to add to enable editing of GPG encrypted files.
" Transparent editing of gpg encrypted files.
" By Wouter Hanegraaff <wouter@blub.net>
augroup encrypted
au!
" First make sure nothing is written to ~/.viminfo while editing
" an encrypted file.
autocmd BufReadPre,FileReadPre *.gpg set viminfo=
" We don't want a swap file, as it writes unencrypted data to disk
autocmd BufReadPre,FileReadPre *.gpg set noswapfile
" Switch to binary mode to read the encrypted file
autocmd BufReadPre,FileReadPre *.gpg set bin
autocmd BufReadPre,FileReadPre *.gpg let ch_save = &ch|set ch=2
autocmd BufReadPre,FileReadPre *.gpg let shsave=&sh
autocmd BufReadPre,FileReadPre *.gpg let &sh='sh'
autocmd BufReadPre,FileReadPre *.gpg let ch_save = &ch|set ch=2
autocmd BufReadPost,FileReadPost *.gpg '[,']!gpg --decrypt --default-recipient-self 2> /dev/null
autocmd BufReadPost,FileReadPost *.gpg let &sh=shsave
" Switch to normal mode for editing
autocmd BufReadPost,FileReadPost *.gpg set nobin
autocmd BufReadPost,FileReadPost *.gpg let &ch = ch_save|unlet ch_save
autocmd BufReadPost,FileReadPost *.gpg execute ":doautocmd BufReadPost " . expand("%:r")
" Convert all text to encrypted text before writing
autocmd BufWritePre,FileWritePre *.gpg set bin
autocmd BufWritePre,FileWritePre *.gpg let shsave=&sh
autocmd BufWritePre,FileWritePre *.gpg let &sh='sh'
autocmd BufWritePre,FileWritePre *.gpg '[,']!gpg --encrypt --default-recipient-self 2>/dev/null
autocmd BufWritePre,FileWritePre *.gpg let &sh=shsave
" Undo the encryption so we are back in the normal text, directly
" after the file has been written.
autocmd BufWritePost,FileWritePost *.gpg silent u
autocmd BufWritePost,FileWritePost *.gpg set nobin
augroup END