Install GMail 4.9 on 2013 Nexus 7 with Android 5 "Upgrade"
There are two ways that I know of. The android package manager won't allow you to install a lower version of the same package. 

Solution 1:
Root your device, install lucky patcher, open toolbox, choose "patch to android", and install the patch that allows the package manager install older versions of the same app

Solution 2:
Using the app ApkEditor, clone the 4.9 apk so that it creates a duplicate app that can be installed in parallel with the default Gmail app. This duplicate app also can't be updated from the Google play store.

If you want, I can provide you with a modified apk to install in parallel, though remember, you shouldn't be trusting a stranger on the Internet. Just comment below.