According to the paygarden FAQ thread, it will only work if it is synchronised with the USA gift card network. Some international cards may work, but it's a company by company basis. Read the FAQ thread below for full information.
The only way to check to see if it will work is to try the code and if it does not give you an offer, it doesn't work. It won't take from your card until you accept the offer so you can feel free to try any gift cards you have to see which would work.
http://www.gaiaonline.com/forum/gaia-blog/paygarden-f-a-q/t.94885665/
Go to the above thread for all the info Gaia has. Others may have opened a guide to show which cards work, but I have not found one yet. This would likely be located in Gaia Guides and Resources.
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