How to go to tab 5 in Safari? (AppleScript)
This maybe an old question but I ran into this same problem.   The solutions so far don't seem to work with Safari 7.x.   This is what I had to do

tell front window of application "Safari"
    set current tab to tab gmailTabindex
end tell


The key is the "front window of".  If you don't have that it doesn't seem to work.