Google is an odd remote as it allows for duplicate file name/folders in the same spot. Rclone since it works with many remote and used a lot of OS file systems does not work with that.
You can choose to rclone dedupe the source and go through the process to rename things so you don't have to those duplicates and move everything over or it would miss things.
With no context, it's hard to tell. 403s are when you hit the API hard and it backs off. 403s are when you hit an upload limit. 403s are when you hit a download limit. 403s are used a lot so you'd have to share more details on what's going on around it and what the actual error is.