Heat oven to 400 degrees. Cut the acorn squash in half length wise (a big knife works best -- be careful!), and scoop out seeds. Place squash cut side down in a baking dish, add a small amount of water (about 1/4 cup), cover with tin foil and place in oven. Cook 30 - 40 minutes, or until very tender when pierced with a fork. Meanwhile, wash, de-stem and chop chard & finely chop or press garlic. Heat oil over medium-high heat in large pan - add garlic and cook for 1-2 minutes. Add chard and saute until reduced in size and wilted, about 5-7 minutes. When the squash is tender, remove from the oven and fill each cavity with sauteed chard. Mix in shredded gruyere with chard. Place filled squash back in oven for a few minutes, just enough to melt the cheese.