How can I use Gmail with HTTP?
There are other questions regarding this issue but they are for several years ago. I think Google has made HTTPS mandatory, but I cannot find any link about this.
You can't. As of 20 March 2014, Gmail is HTTPS-only.
From the official Gmail Blog:
Starting today, Gmail will always use an encrypted HTTPS connection when you check or send email. Gmail has supported HTTPS since the day it launched, and in 2010 we made HTTPS the default. Today's change means that no one can listen in on your messages as they go back and forth between you and Gmail’s servers—no matter if you're using public WiFi or logging in from your computer, phone or tablet.