Replacing the flavor of soy sauce will be close to impossible. But I think that a combination of Worcestershire sauce, fish sauce and a liquid from soaking dry mushrooms could come close in terms of complexity and "umaminess".
You might consider Worcestershire sauce, which is already used in China & Japan:
https://en.wikipedia.org/wiki/Worcestershire_sauce#China
Check on the ingredients where you are, as formulations vary by country and by brand. But some are gluten-free, e.g. Lea & Perrins in USA, Heinz in Canada.
as @SusanW has mentioned, coconut aminos is probably your best bet. Here's a review with a description of the flavor of coconut aminos. Sounds like you would probably need to add salt. Best of luck, http://byebyegluten.blogspot.com/2010/01/gluten-free-soy-free-soy-sauce.html
She needs it to be soy free. You can purchase tamari made from garbanzo beans, but because it's a byproduct of making miso, it has to be ordered online from companies that make garbanzo miso like South River Miso. It's delicious, but they don't always have it available.
It will be a little different flavor wise, but I like to use fish sauce - salty and umami, and should be safe. I know liquid aminos are also a favorite of paleo folks, as someone mentioned elsewhere, but I'm not a huge fan.
Lots of Paleo people use this. It's quite good.
http://www.amazon.com/Coconut-Secret-Original-Soy-Free-Seasoning/dp/B00LWQLCJ4/ref=sr_1_1?ie=UTF8&qid=1447899863&sr=8-1&keywords=coconut+aminos
15 Comments
https://en.wikipedia.org/wiki/Worcestershire_sauce#China
Check on the ingredients where you are, as formulations vary by country and by brand. But some are gluten-free, e.g. Lea & Perrins in USA, Heinz in Canada.
http://www.amazon.com/Coconut-Secret-Original-Soy-Free-Seasoning/dp/B00LWQLCJ4/ref=sr_1_1?ie=UTF8&qid=1447899863&sr=8-1&keywords=coconut+aminos