Question

a gas filled balloon moves up. is the upward force acting on it larger or smaller than the force of gravity

Answer 1

the upward force acting on it is larger  than the force of gravity

Answer 2

the balloon is moving high  because of the unbalanced force acting on it. When balloon is filled, it will get gas inside it. The property of gas in that it is having more energy and will be in motion continuously . This will make a new force inside it which will be more than gravitational force. 
The more the gas is present, the more the balloon can fly up cause then the upward force present will be high.