@chrisg, I think that post that @jonsully linked to would be useful in your case. From experience, ‘Not found’ typically points to a permissions issue. Can you try the steps mentioned in the post and see if that helps? Basically, the following:
- disabling Git Gateway in the Netlify UI,
- revoking Netlify Auth access in my personal Github account (not the one where the repo lies) and then
- enabling Git Gateway in the Netlify UI again
has solved the problem
Let us know if that helps.