Chrome / Safari / FireFox are all freeze their user agent string so you can not know operating system version, device type and browser build number (not the major version!) anymore. This is done to make fingerprinting more difficult.
Here is some Chrome documentation:
If you need to access such high entropy data you need to use a new API. GWT itself should not be affected by these changes since you can still sniff if it is Chrome / Safari / Firefox and which major version it is.