I'm willing to put money down the gas light has a current bleed, causing the LED to be constantly lit. As for the gauge, I'd suspect the plug in the back may not be seated fully or been forgotten. Unless you were less than gentle and possibly damaged the ribbon wire. At any rate, swapping in an LED won't cause the gas gauge to stop functioning so I wouldn't suspect these two issues to have a common denominator - unless perhaps Toyota made the gas light go off if the gauge stops being responsive? That doesn't sound familiar to me, but I wouldn't be surprised either. I'm almost positive the low fuel light is one of the common positions with a current bleed, causing many to stick with incandescent bulbs
Disconnecting the battery terminal would not have made a lick of difference; there is zero reason to have it disconnected when replacing a bulb and similarly no reason why failing to do so would have this result