As I understand it, if you have a paper trail to any relative - out as far as a 3rd cousin - who is on that grandfather's side, you confirm the entire string of ancestors up to the common ancestors. For a 3rd cousin, you would confirm your grandfather, great-grandparent, and gt-gt grandparents, all at once. I believe you have to show that you match that person on the same segment of the same chromosome.
Or you can use more distant relations, but you need 3 of them, from different branches.
When I last looked at it (months ago), the rules appeared to have some flaws in how they're written (it's complicated). Then I saw a post (that I wish I had saved) that seemed to give a different story, vs the whole path having to be on WikiTree, with the matches on GEDmatch. So I'm not sure exactly where it stands now, but I'm pretty sure you're not just limited to having to have your 1st cousin tested.