autoload/*.vim <-> plugin/*.vim

gistfile1.vim

function! s:open_pair_file()
  let path = expand('%:p')
  let pairs = [
        \ ['/autoload/\([^/]\+\).vim$', '/plugin/\1.vim'],
        \ ['/plugin/\([^/]\+\).vim$', '/autoload/\1.vim']
        \ ]
  for [pat, sub] in pairs
    if match(path, pat) != -1
      execute 'hide' 'edit' substitute(path, pat, sub, '')
      break
    endif
  endfor
endfunction

nnoremap <Space>p :<C-u>call <SID>open_pair_file()<CR>