I am assuming Gmail caches the images when you upload, image upload being an expensive operation per se; when you uploaded so many at once, it must have triggered a rate limiting mechanism which stopped requests from you until the cooldown window kicked in.

Why would they add a rate limiting method? It makes sense for every product to have rate limiting beyond fair use, otherwise it makes it susceptible to denial of service attacks by attacking expensive APIs with high volume.

Why did the rate limiting mechanism blanket limited Gmail? I am assuming they either just have a global threshold or you breached both API and global threshold at the same time.

Should they have informed you? Maybe the message was in HTTP code.

Again, no way for me to verify this. These are just my assumptions from building rate limiting solutions in the past. Every service should have rate limiting mechanism, probably with better user messaging I guess.