Drunk, homeless and hopeless. That was how people that knew Grahm described him. His home was wherever he went, sleeping on back alleys, public toilets, park benches, you name it and he probably has spent a night there before. When the infected took over, he saw it as an opportunity. Everyone was equal again. Doesn't matter if you were rich or poor, when humanity fell, everyone only wanted to survive.
Spending much of his life salvaging rubbish for useful items and food turned out to be a very useful skill in the post apocalyptic world, Grahm was able to quickly search and loot places abandoned and got himself quite a inventory. Now a roving trader, he travels Chernarus trading odds and ends that he finds.