As long as you don't forget your password, and Google doesn't stop drive, your passwords should be secured...
Well, that's the risk,
if you don't forget your password.
And writing passwords in some papers offline is much better than uploading your passwords/private keys/phrases online in my part.
It has lesser risk too since gmail/emails/websites are too risky from hacking than saving it offline, you only have to worry is from the people around you, those people who knows that you have some wealth you saved on some of your notebooks or any kind of papers lol. Tho I save some private keys online but they are encrypted with password in a zip file and I have also a copy and save them offline.
Oh well, I don't feel comfortable about the offline method. But it is still fine if that's what works for you.
How do you encrypt the zip file? I haven't tried that before.
Archiving software normally have encryption options when you create the archive (zip, rar, 7z etc.). Try 7zip, open source and free and lots of archive formats.
My personal way is txt file, archived, encrypted and then renamed as a random file format. Like say, a batch file or system file. Hide it in a system folder with lots of other files so it doesn't stick out.
Password and private key strings also inserted within another string... So even if discovered, still needs to be figured out.