That is a text field feature, not a keyboard feature. Google controls all the text fields for gmail. Both in the web version and in their native apps.
[0] https://developer.mozilla.org/en-US/docs/Web/HTML/Global_att...
Also.. not sure what safari/iOS did in their early years with keyboard password entry capitalization... but if they did auto capitalize... since Apple is so good at saving profile info across new installs/os updates... I imagine there would be a large portion of old apple users with perma-capitalized passwords out there as well.