Purgatory City by Rebecca Royce

Play the Game or die.
Alone in a desolate world ravaged by war and alien invasion, Eileen Paine offers herself to indentured servitude in Purgatory City—a chance at a different life.
No one knows what happens beyond the gates, but it has to be better than the peril in the Blastlands. Nothing could have prepared her for the darkness, the tests, the changes, or the people she meets. Least of all being changed and controlled by the aliens in power.
With no guarantee of success, eliminations happening frequently, and the threat of potential allies becoming enemies, does Eileen have what it takes to survive? And when her heart gets involved, can she live with what she has to do to escape Purgatory?