You should begin your presentation by discussing the respective contexts of the two works. The Epic of Gilgamesh is an ancient Mesopotamian epic that was based on the story of Gilgamesh, a King of Uruk, who ruled sometime between 2800 and 2500 according to Sumerian king lists. The epic was gradually embellished over many centuries from circa 2100 to circa 1200 BC. Sir Gawain and the Green Knight is a fourteenth-century romance by an unknown author extant in a single version. A major and important difference between how death is viewed in the two is the different religious backgrounds.
Mesopotamian religion was polytheistic and thus had many different gods. One of the main differences between gods and humans was that gods were immortal and humans were mortal. People with partially divine parentage lived longer than other mortals and had some godlike characteristics, but the gods tended to be jealous of mortals who did not know their place in the world, which was to worship and serve the gods. After death, humans descended into the netherworld, a dark, dismal, and unpleasant place, where they existed as spirits. Although the netherworld was not divided into a heaven and hell—it was neither a place of reward or punishment—it was not particularly pleasant. Thus, we can sympathize with Gilgamesh's desire to avoid death. The outcome of Gilgamesh's quest for immortality illustrates a belief we find in other works of the period, namely, that any quest for immortality is futile and that people should focus on making the best of their current lives.
Gawain and the Green Knight is a Christian poem, filled with Christian imagery. The key difference is that rather than this life being the best one and the netherworld a sort of dim shadow of mortal life, for Christians, this life is preparation and testing for an afterlife in which those who lead good lives are rewarded and those who lead evil lives are punished. Death of the body is not a thing to be avoided. Even if the body dies, the soul is immortal. Thus, the issue in the poem is not a quest for immortality of the body, but testing and training Gawain so that his soul will be able to go to Heaven. There are many images in the poem of Resurrection and testing, suggesting that it is a deeply religious poem meant to impress upon people that Gawain's physical strength is less important than his moral character.