The Snapmaker originally uses a 0.4mm nozzle, there is one replacement hotend with the same nozzle-size in the package.
A slicer is a program like luban for 3d printing.
It prepares your model, with your settings, to g-code, so your printer could understand.
There are many slicers on the market, “Cura” is the most used one, i guess.
If you are not familiar with this and new to 3d printing i would suggest you to simply print your gears and see if they work. If they are coming out worse than hoped, you may should give it another run some time later, when you got more familiar with 3d-printing.
Hope this helps!