Answer (1 of 6): I'll answer as a former engineer at Google, who has worked with both Closure and GWT within the company and participated in porting his project from one to the other. I don't claim to know what the G+/Emerald Sea team ultimately chose Closure, so this is still an educated guess a...
![Why didn't Google engineers choose GWT (formerly Google Web Toolkit) for Google+ frontend? Closure Library (Google) is used on Google+ UI.](https://cdn-ak-scissors.b.st-hatena.com/image/square/5f6ea7c1d18d484554d47fd2be703b264da33773/height=288;version=1;width=512/https%3A%2F%2Fqph.cf2.quoracdn.net%2Fmain-custom-t-786-600x315-xrkeuvcslzgowwvtlbzqzkuiwipwjliz.jpeg)