decode-gpg.lua

local gpgGroup = vim.api.nvim_create_augroup("customGpg", { clear = true })

vim.api.nvim_create_autocmd({ "BufReadPre", "FileReadPre" }, {
  pattern = "*.gpg",
  group = gpgGroup,
  callback = function()
    -- Make sure nothing is written to shada file while editing an encrypted file.
    vim.opt_local.shada = nil
    -- We don't want a swap file, as it writes unencrypted data to disk
    vim.opt_local.swapfile = false
    -- Switch to binary mode to read the encrypted file
    vim.opt_local.bin = true

    -- Save the current 'ch' value to a buffer-local variable
    vim.b.ch_save = vim.opt_local.ch
    vim.cmd "set ch=2"
  end,
})

vim.api.nvim_create_autocmd({ "BufReadPost", "FileReadPost" }, {
  pattern = "*.gpg",
  group = gpgGroup,
  callback = function()
    vim.cmd "'[,']!gpg --decrypt 2> /dev/null"

    -- Switch to normal mode for editing
    vim.opt_local.bin = false

    -- Restore the 'ch' value from the buffer-local variable
    vim.opt_local.ch = vim.b.ch_save
    vim.cmd "unlet b:ch_save"
    vim.cmd(":doautocmd BufReadPost " .. vim.fn.expand "%:r")
  end,
})

-- Convert all text to encrypted text before writing
vim.api.nvim_create_autocmd({ "BufWritePre", "FileWritePre" }, {
  pattern = "*.gpg",
  group = gpgGroup,
  command = "'[,']!gpg --default-recipient-self -ae 2>/dev/null",
})
-- Undo the encryption so we are back in the normal text, directly
-- after the file has been written.
vim.api.nvim_create_autocmd({ "BufWritePost", "FileWritePost" }, {
  pattern = "*.gpg",
  group = gpgGroup,
  command = "u",
})

-- Return an empty table to satisfy plugin loader requirements
return {}

Great! Though I needed a small fix, see https://gist.github.com/rodolfoap/a50b5fa7758c302ad74fcafe93c46efc

Keep it up :)