Google offered custom domain support to individuals decades ago, and slowly pivoted that offering to become Google Workspace. Their systems don't differentiate between personal users with a custom domain and giant-ass corporations. As such, many Google products reject logins from custom domains until the offering is beefed-up to meet enterprise demands.
Music and Photos started this way, but eventually upgraded to allow custom domain accounts. Stadia was always gmail-only. Google One too. Home/Nest kind of works with a custom domain, but you can't share your devices with family members. There was a whole year there where it stopped allowing you to set reminders from a custom domain, and it still doesn't reliably work.
It's been a long time since I've set up a Google account - you might be able to use a non-gmail domain now and still be treated as a person (e.g. an iCloud user who wants to login to non-gmail Google products), but stay away from any sort of Workspace account as an individual.
Music and Photos started this way, but eventually upgraded to allow custom domain accounts. Stadia was always gmail-only. Google One too. Home/Nest kind of works with a custom domain, but you can't share your devices with family members. There was a whole year there where it stopped allowing you to set reminders from a custom domain, and it still doesn't reliably work.
It's been a long time since I've set up a Google account - you might be able to use a non-gmail domain now and still be treated as a person (e.g. an iCloud user who wants to login to non-gmail Google products), but stay away from any sort of Workspace account as an individual.