Gouache is a good training medium for oil, even though they're not at all similar. It's great training for oil because gouache is opaque, and can be applied thinly or thickly, which is similar to oil. What's opposite to oil is it dries very quickly, unlike oil that takes a while to dry. But it's a good thing because it builds good habits of painting in decisive shapes. Oil has a long time to be manipulated so students tend to over edit and don't commit to a shape. Finally, even though gouache dries quickly, if you applied it thick enough, it can be reactivated with oil, so you are able to blend the edges if you'd like. Jeff Watts, Proko's teacher, uses gouache to sketch out an oil painting during the planning phase.

I know some schools teach gouache as a precursor to oil. It's rather different in nature, but it's good training since it forces you to make decisive painting and rendering choices, while also being more cost efficient. Gouache has its own learning curve as well, but I think it's quite worth the effort given its usefulness and versatility. Best of luck :)

I don't think gouache is an alternative, as it is opaque. Acrylic springs to mind, possibly using an medium to extend its open time. Acrylics dry fast in comparison with oils, though: it needs some practice to get used to it.