How can I install Gmail on an iPad os 5.1.1?

I do not think this procedure has changed too radically since iOS 5.1.1.

You setup mail by going into the Settings App, under Mail, Calendars, Reminders.

iOS has a list of Mail host options that you can setup.

GMail should be one of the options to setup.

You tap on the GMail option and just setup your Name, Gmail address and password.