Allons-y was a phrase which some kids who thought they were clever used to say at my school back in the early 70s -- which is one reason I cringed so much when the Who writers (showing their age and background) had the Doctor say it. These same schoolkids went on to 6th form college where they thought it equally witty to call a cup of coffee a "caffeine attack".
As others have already commented, it was used as a jocular way of saying, "Let's go," but in a kind of "unto the breach" (another overused cliche) way.
Not so much Gallifrey as Nerdshire.