When setting up a gmail account you are giving two options “email only” or “email, contacts and calendar.” If you select the email contact and cal, you get to select push. If you select email only it sets it up smtp, which only gives you “every 15 minutes.”
select the second option and just leave contacts and cal unchecked and all will be well.
On a related note. I could have sworn on my WP7 device Gmail was able to use Push, but on my WP8 i only have the option to check every 15min?? Am i nuts?