Maybe it's because I live in a country where e-mai isn't really used that much for personal communication, but wouldn't this mainly be a gmail issue? If mails I wanted ended up in the spam folder I'd not use gmail. I mean, I pay for protonmail, so I wouldn't use gmail to begin with, but if mails I wanted ended up in my protonmail spamfolder and I couldn't do anything about it, then I'd switch away from protonmail.