Is the HP Z420 pc good for Gaming? (like cod bo2, minecraft and more)?

That depends on what graphics card you choose for it. Quadro K1000 or FirePro M5800 and similar cards would play those games just fine at high settings, BUT for more demanding and recent games they would only get you by at medium settings.

If you want to game on high settings in future games you'll have to pony up for the newer and more expensive professional cards (assuming you need a workstation).

FYI, workstation grade PCs are usually inefficient for gaming compared to consumer PCs, not only in hardware design, but also per price point. If you're just looking to play games then I recommend you look for PCs with GeForce or Radeon cards instead.