If you click on any email address link on the web then outlook or some other program in your Computer pops up which you may not used often, instead if you want Gmail compose window to open with pre-filled email address you clicked on in the To field-you can get this working for Gmail in Chrome thanks to HTML5.

Set Gmail in Chrome as default mail client in Windows

From now onwards whenever you open  Gmail in Chrome a message appears on top of the page asking you to “allow Gmail to open all email links”   if you click use Gmail, Gmail will be set as default mail client in Chrome and clicking on any email links in any application on your Computer will open a fresh Gmail compose window.

allow gmail to open all email links message

gmail compose window opened in a pop up

Source:Gmail blog