Hello and a warm welcome to RootsChat.
Someone may correct me but I believe ATIP is only available to Canadian citizens (as you mentioned), plus permanent residents or people residing in Canada. It doesn’t look like there is any other way to access the records. If you aren’t in Canada you could try email or a letter to the ATIP branch and explain what you are looking for. You never know what might happen. Info can be found in the ATIP section (couldn’t get it to copy and paste properly for me).
Meanwhile, if you would like to give some onfo about your great granddad we could try and find him in the censuses, etc.
polarbear