How can I make it so that `gopls` runs when I am editing `.tmpl.html` files?

I would like to have some intellisense/autocomplete for Go template files, but nothing I have tried has done anything. I have tried giving the files the .gotmpl extension, I’ve looked up how to add filetypes to the gopls server (I am using lsp-config), I have tried just sticking this autocommand in my lua config (I did edit it a little:

        pattern = "*.tmpl.html,*,*.tmpl.yaml,*.tmpl.yml",
        callback = function()
            local buf = vim.api.nvim_get_current_buf()
            vim.api.nvim_buf_set_option(buf, "filetype", "gotmpl")

And I have even tried running :setlocal filetype=gotmpl, which started the server, but there wasn’t any completion.

How can I have autocompletion for go template files?