Based on https://gist.github.com/mrash/0f12560983a8c2888c5f

decode-gpg.lua

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

-- autocmds execute in the order in which they were defined.
-- https://neovim.io/doc/user/autocmd.html#autocmd-define

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

        vim.cmd("let ch_save = &ch|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

        vim.cmd('let &ch = ch_save|unlet 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'
})

When I open a .gpg file, It displays the contents of file unencrypted as expected, but at the bottom it says 'Press ENTER or type command to continue'. If you hit ENTER or press any other key, I see the following error:

Error detected while processing BufReadPost Autocommands for "*.gpg":
Error executing lua callback: vim/_editor.lua:0: BufReadPost Autocommands for "*.gpg"..script nvim_exec2() called at BufReadPost Autocommands for "*.gpg":0: Vim(let):E121: Undefined variable: ch_save
stack traceback:
        [C]: in function 'nvim_exec2'
        vim/_editor.lua: in function 'cmd'
        /Users/me/.config/nvim/lua/plugins/decode-gpg.lua:30: in function </Users/me/.config/nvim/lua/plugins/decode-gpg.lua:24>
        [C]: in function 'nvim_exec_autocmds'
        .../AstroNvim/lua/astronvim/plugins/_astrocore_autocmds.lua:171: in function <.../AstroNvim/lua/astronvim/plugins/_astrocore_autocmds.lua:169>

Then if you press any key, the text is cleared out.

EDIT: I made some small changes to make it work for me: https://gist.github.com/nickali/89f3743e305db015d0f3ad4ffd325ccb.

awesome, thank you @nickali