From Beowulf we know that Hrothgar's meadhall is Heorot. A reader of Gardner's Grendel does not encounter that word. Instead, Hart is the site of the monster's "idiotic war" on the king.
Beowulf (Gummere translation) gives us this description of the hall:
Notes to this translation tell us about the hall:
The American Heritage Dictionary tells us that the term "hart", meaning " a male deer, especially a male red deer over five years old," comes from the Old English "heorot". So there we have it. Gardner's choice is not so odd after all.
But what about Hrothgar's choice? Why would he decide to name his "magnificent hall" after this wild creature? We know that in Gardner's novel it is an "antlered hall" (46); that is, according to the Gummere translation notes, "so called from decorations in the gables that resembled the antlers of a deer." It is likely that one or more sets of hart's horns are mounted in the hall.
Does this animal have some ancient connection to Hrothgar's tribe? Does Hrothgar wish to emulate some quality of the hart - its speed, elegance, strength, or sexual prowess (the horns, after all, are biologically related to the animal's reproductive cycle)? Maybe it is a symbol of simple admiration: a king might wish to be as powerful as this noble creature.