How thick should gravel bed be?

depends, with plants a mixture of sand and fertilzer like eco complete or red sea is best imo. gravel should be at least 1 1/2 inches in depth

I have 2"+ on all my planted tanks. Its hard to root them other wise.

I think its mostly however much you want within reason. No more than 3".

Some people pile up the gravel to be 6" or so in one small area which is ok too. It makes the tank a little more interesting and seem like the cross section of a stream opposed to something artificial.

Sand, flourite, and gravel are most commonly used. I like to mix sand and gravel for a realisitc stream type substrate. Flourite is also very good for growing plants.

i think a question should be what kind of plants are you looking at