There are a lot of different gouramis, though! Some are only an inch or two in size (sparkling gourami) and some (giant gourami) over two foot! Some (three spot/opaline/ moonlight) can be also be very aggressive.
Volume isn't really that important when you're thinking about stocking. It's all about surface area and swimming room, so you're going to have to stock it more like a 10g than a 30g.
Measure it up and we can think about species.
Hardness isn't the same as pH either. Do you get a lot of limescale in your kettle/showerhead?