Hi @rsolva!
Just put the code posted above in init.vim (or maybe a linked autogroups.vim file).
Then open a Hugo/Go template file in Neovim, press : and enter set ft?. This should prompt:
filetype=gohtmltmpl
Then you can install the whole plugin or just download the needed files and put them in the corresponding folders in e.g. ~/.config/nvim/.
I use these files only:
~/.config/nvim/ftplugin/gohtmltmpl.vim
~/.config/nvim/indent/gohtmltmpl.vim
~/.config/nvim/syntax/gohtmltmpl.vim
Good luck!