That's right! I have made several gears with my horizontal mill and index
head. You need a gear cutter for doing spur gears which are available from
many tooling suppliers. You can make reasonably good worm wheels with simple
shop-made cutters or even with taps. One of these days I mean to set up a
gear train to the index head and try hobbing some gears.
The book "Gear Cutting Practice" by Colvin and Stanley (1937) is a good book
to get your feet wet learning the terminology and techniques. You can order
it through Lindsay. Another good one is "Gears and Gear Cutting", by Ivan
Law, which is more appropriate for home shop machinists with basic
equipment. It shows how to make shop-made gear cutters, among other things.
I ordered a copy from Amazon, it is published in the UK.One day I will make
the form-relieving tool shown there and make some interesting cutters.