Neither USDC nor GUSD nor even USDT is backed by "physical assets" since they are backed by dollars (so they say but I doubt it) and dollars are not physical assets, they are digital as much as regulate bitcoin, the amount of banknotes in the world are much less than the amount of dollars in the world which means there are digital dollars that is not printed.

Physically backed things are like gold coins where one coin equals one ounce of gold or something, kinda like tether but instead of dollars you literally have golds in your deposit directly and not digitally, things you can literally hold in your hand. Gold is something you can literally hold in your hand whereas dollar is something you put into your bank account. However considering bitcoin and crypto is digital, I doubt there is a need to actually back it with something physical.