Honestly, there are a good number of places and things that can be used as a gateway symbol. Basically anything that can represent (or outright be) a 'tween (IE; between one thing and another) can be used symbolically as a gate.
The most obvious is a doorway, mirrors (they reflect reality), the surface of water, natural openings like caves, constructed archways like stone arch bridges, windows, the horizon, fog (between light and dark), etc. Of course some are more practical in a figurative sense. for example the horizon is technically impossible to reach, but one ca meditate on the idea.
As far as crystals are concerned, geodes could be used as a symbolic reference similar to the opening of a cave. And scrying stones as well- teardrop shaped polished quartz is pretty effective for looking through a gateway. Especially if it has a few little imperfections inside to stare at.
Usually they are used symbolically. Meditation and visualization works. I believe in magic they are called gateway rites? (Someone correct me if I am wrong please @_@)
If you have a mirrored surface, a body of water, or some sort of scrying stone or geode with a narrow opening you can stare into, you can meditate while looking into the item and imagining yourself being drawn into or through the opening or surface. Imagine the threshold of the item or opening as a gateway you can step across or through and let yourself sink into the idea.
The best times to do this would of course also be 'tween times. Like dawn and dusk when day and night are transitioning. Or while within a heavy fog or mist. Or if there is a natural doorway in your area like a cave entrance, or natural archway you could sit in that place while you meditate.
The process, to me, is very similar to practices of astral projection. It just has a more specific goal in mind like reaching the spiritual plane, or a plane directly associated with what you are using for your gateway.