Mea culpa, I missed the word "gmail" in the OP's description.
With my Gmail accounts where I am using Gmail's MSA (as opposed to 3rd party MSA servers), I am not sending many messages, so, I haven't experienced it as much. Usually, when traveling by air, I am being blocked by Gmail's IMAP servers at the new place until I "authorize" access by logging in in a web browser, or jumping through other G-hoops.
I am guessing that in case of restrictions for sending messages from public-Wi-Fi places, Gmail is trying to limit SPAM.
When you are sending messages from a non-public IPs (your home, work, etc.), - you are less likely to be a spammer.