I don't think you can get benzoin very easily in the US and it isn't approved as a preservative.
I am not very hip to GSE either. The companies that DO use natural preservatives use a very complex process that I cannot possibly describe...burt's bees does that for some of their products.
There are two preservatives that work very well..and if you ever intend to sell your cream that's what you will have to use as I do not think GSE is approved as a preservative in creams, creams qualify as "cosmetics" and thus have more regulation (eerr...I think the FDA does that)
phenohip and germaben II are both good preservatives but neither are considered natural. But they are the most widely used among those who make stuff like that to sell.
They can be easily found at most soapmaking websites. Phenohip works well if you mix your preservative in when the cream is still pretty hot. If you use Germaben the cream has to be warm..if it is hot it will jack with the preservative.
If you are really wanting to go the natural route, and just use it for you then don't use a preservative and keep your cream in the fridge.
It lasts about six months that way.

Without a preservative or being in the fridge it won't last long at all.
But like plantspirit said you'll need distilled water also, I have really hard water and have to be extra careful when washing my utinsels I use for things like that or I get weirdness.