So assuming a 20 Stat, 1 attack at +7 against AC 15 Target
Has a 65% to hit, jumps to 87% with Advantage. DPR 8
With GWM, without Advantage, you hit only 40% of the time. You hit 64% of the time with Advantage, and your DPR increases to 16.
Elven Accuracy increases your chance to hit to 95%...