Open URL in Google Chrome. Switches to tab if already open or creates a new one.

open_url_in_chrome.txt

# source: http://superuser.com/a/692131/424541

on is_running(appName)
    tell application "System Events" to (name of processes) contains appName
end is_running
set chromeRunning to is_running("Google Chrome")

if chromeRunning then
    tell application "Google Chrome"
        set i to 0
        set j to 0
        repeat with w in (windows)
            set j to j + 1
            repeat with t in (tabs of w)
                set i to i + 1
                if title of t contains "Gmail" then
                    set (active tab index of window j) to i
                    return
                end if
            end repeat
        end repeat
        tell application "Google Chrome"
            activate
            open location "http://mail.google.com/"
        end tell
    end tell
else
    tell application "Google Chrome"
        activate
        open location "http://mail.google.com/"
    end tell
end if

There's a minor bug here: you need to reset i to zero again for every window, not just at the beginning.