Knee-Deep in the Dead — Dafydd ab Hugh; Brad Linaweaver

The Gates were there on Phobos when mankind first arrived. Inert, unyielding, impossibly alien constructs, for twenty years they sat lifeless, mute testaments to their long-vanished creators, their secrets hidden. Then one day, they sprang to life...

Meet Corporal Flynn Taggart, United States Marine Corps; serial number 888-23-9912. He's the best warrior the twenty-first century has to offer, which is a damn good thing. Because Flynn Taggart is all that's standing between the hell that just dropped in on Mars and an unsuspecting planet Earth...