Forwarding. So, I haven't actually had the SPF issue you mention. I stopped using that setup about 2 years ago in favour of my proper gsuite account and I believe that, at some point since that time, Google's setup changed causing this difference in validation. I had heard of some setups being grandfathered in. I was at the time using the "additional account" email setting in gmail.

So in the emails I would send, my @gmail.com would actually appear in the headers, even though the From: was my custom domain.

I don't know if this setup is still available.