How many gallons of fuel does a jet use just on takeoff?

It all depends on how big a jet we are talking about. As an example something like the smaller 747s have four engines of 50 thousand pounds thrust each. Takeoff lasts about 30 seconds from standstill. To generate a pound of thrust for an hour at sea level at low (or no) forward speed takes about 0.4 pounds of fuel.

So,

4 * 50000 * 30/3600 * 0.4 = 667 pounds of fuel

This is roughly 100 US gallons.