Is fresh ginger a good substitute for galangal in a Thai Green curry? (I cannot get fresh galangal here anywhere)
Would I use less or more ginger than galangal quantity?
Use equal amounts of ginger to substitute for galangal. And yes, it is the best substitute available from your average non-Asian grocery store.
If you can get your hands on dried, powdered galangal, however, you can do better. Add about half the amount of ginger, and around midway through cooking add a teaspoon of powdered galangal for every tablespoon of fresh in the recipe. The combination of fresh ginger and powdered galangal will be very close to fresh galangal in flavor.
Also, if you make a field trip to an Asian market, peeled galangal cut into chunks freezes quite well.
It won't be quite the same in flavour, but it is near enough that the food will still taste good and the combinations of flavours will work well.
Unfortunately, I too have had only store-bought ginger since my galangal died...