“Green garlic powder"? Do tell, I've never heard of it.
In Asian cooking, garlic is used in many forms, not just the bulb/cloves. So it is not uncommon to use green garlic, which is the garlic stem and leaves (including roots) before the bulb ripes into cloves. They look exactly like spring onions (UK term) and just to clarify, I mean the ones that look like mini leeks, long and thin and you eat the stems, leaves, immature cloves and often the roots as well....
A company just down the road from where I live (actually on the same road) grows and sells garlic and they have reintroduced many alternative forms of it, including the green garlic in fresh and powdered form.
Another garlic company close to where we first lived when we moved to Australia, makes black garlic, also delicious. Trouble is, it is that nice, I eat it whole from container with it never making it to the dish! Lol.
As MG mentions , you could probably make your own if you grow your own garlic, harvest before the bulb fattens and happen to have a dehydrator (probably an Excalibur or a professional one). It needs to be dried very slowly at a very low heat, then when totally dry, ground to a powder. I do grow my own and use them fresh from the garden as fresh green garlic, but I don't make my own green garlic powder despite having an Excalibur dehydrator. Don't see the point...
I'm just waiting for their garlic season to start so that I can stock up on fresh garlic bulbs after I lost most of my garlic to the native wildlife earlier this season.