Not bad.
It's a passable shooter. The graphics are pretty polished - the style leads me to believe probably ripped from a Korean MMO, but they work nicely either way. Props to you if that actually is your original work, too.
Your gameplay really isn't anything new or ambitious though, and there are some balance issues you could do to redress. Notably: The final weapon -sucks-. Just some quick testing confirms it takes the same number of gatling bullets as crossbow bolts to kill a basic zombie. If the one is more powerful than the other it's at least not by much, but it's possible (and in fact, really easy) to achieve a vastly superior rate of fire with the crossbow. If you're going to throw in an automatic weapon, it needs to have a high enough rate of fire to actually compare to the semiautomatic ones. You could consider limiting the rate on the semiautomatic weapons instead of going off as fast as the player clicks as well, which would actually give us a reason to use the machinegun.