GDAX: Balance and placing a limit order I had some money on balance in my GDAX account.
I placed a limit order on a price below market price for the whole amount I had, and ... my balance now shows 0.00

The limit order I placed is still open, but my balance decreased. This is different to for example Kraken, where your balance remains intact until order is executed. 

Is that how GDAX work, and as soon as I cancel my order, my balance will be restored to original value? Is this typically normal for exchanges? (I only used Kraken until now)
Every exchanges have that in place.
This is to prevent abuse. If they don't reserve the funds when people place orders, anyone could have placed infinite amount of orders, especially stop-loss orders.
Perhaps in Kraken the "balance" doesn't reflect that, but you can try to place double orders and see for yourself.