1) What kind of gears - straight spur, helical, bevel, worm, crown, rack, hypoid, et al?
2) What purpose - drive CNC, drive Simulation, Look Good?
You can either create the teeth using datum curves driven by equations, which is pretty easy, or try using Inkscape which has an involute spur gear creator and export the profile from there.
If you haven't done any work with involute gears before, there are some videos on YouTube from the Fellows Gear Company that are short and highly informative.
The trickier part is if you are creating gears that have few teeth or some other special case. If so, then consider looking to hire a consulting firm that has some experience in gear and plastics. It's a common enough problem that there are some good choices. There are an infinite number of ways to tweak the design of the teeth and there's not enough time for me to type them all out.