While, Google will not necessarily end Gmail altogether for users of Windows XP and Vista, as the basic HTML version of Gmail may continue to work on the platform, but updates will definitely stop, which means users will miss out on new features and bug fixes.
According to Google, starting on Wednesday, February 8, a banner notification will appear at the top of Gmail recommending to users to upgrade their version of the browser, which obviously users of the outdated OSes won't be able to update as they're limited to Chrome 49.
Even Microsoft had previously warned as far back as in April 2015, that these systems are no longer supported by the company, and strongly recommend that users should migrate to more secure and supported systems.
Albeit, not every single user of Windows XP and Vista also uses web-based Gmail and Chrome, as there are possible ways around the shortcomings, like switching to a desktop program for email such as the built-in Outlook Express on XP.
Google does not typically announce discontinuation of support for older versions of Chrome browser, but this exception was made given the expected impact on Windows XP and Vista users and known security risks.