Gunner Z is a sad state of affairs. The core gameplay (on-rails shooter) could be fine on its own, but it's bogged down by so much microtransactions and hooks to try and make you spend money that everything in this game is terrible. I had some fun with it, but it wasn't worth my time.
You are in a vehicle that moves automatically on a set path, you move a camera attached to a gun and shoot zombies and other threats. You have a bunch of weapons and subweapons and you get money (and other premium resource BX) for killing enemies and completing missions. This game gets too hard way too fast, jeeps shoot guns at you with cold efficiency, drones that fly in really annoying patterns, zombies are slow but come en masse from all directions at some points and if they get too close and start wailing on your jeep, you just bounce around and can't aim well, a very small number of hits later, it's game over...
And the game will go "pay us some money if you're having trouble!". You can pay to continue, you can pay to double your stats, you can pay to revive automatically once per level (it's 25$) you need to either wait or pay for upgrades to be applied to your vehicle, you need to wait or pay for your vehicle to be repaired if it gets destroyed during a mission and you're still constantly spammed to buy things. Unwanted pop-ups here and there. It's all pretty terrible.
Otherwise it would be fine, you have plenty of things to upgrade for your primary and secondary weapon with a bunch of types (shotguns, machineguns, rocket launchers) to choose from, you can equip special IED detectors and increase the range of your zombie radar, improve your armor and suspension, all things that would be acceptable if you didn't have to then pay for it to be completed before your attention with this game fades away.
Don't play Gunner Z, don't pay for the cash doubler (and I love cash doublers!) and don't expect anything from this nonsense. It's frustrating and will try to get money from you.