How can I get Aperture to use gmail for emailing photos?
To make "share by email" buttons on your Mac (system-wide!) open up in Gmail—instead of the Mail app—follow these instructions:

-Download and install Google Chrome, if it's not on your Mac already.

-Open the Mail app--yes, the stock Mail app.

-Press "Mail" on the menu-bar, in the top-left of your screen, and hit Preferences.

-Make sure you're in the General pane, and press "Choose default Email Reader". 

-Choose Google Chrome from the dropdown menu.

Now your system-wide default email viewer is Gmail in the web. Let me know if this helped you!

Source: http://osxdaily.com/2014/05/06/change-default-mail-app-mac/