Does anyone know if it's possible to backup directly to the Gmail storage?
It seems to me that Gmail storage is different from Google Drive.
Background: if you ran out of storage on Google Drive, you can still upload attachments in Gmail which to me tells me that it must be a different storage.
... some people literally have over 100TB of data on Workspace (google drive) and you are worried about running out of space?
Secondly Google cloud data for pictures and gmaiæ got heavily redused no so long ago: How your Google storage works - Google One Help.
In short; no. Its not possible with rclone to upload to your regular account storage