Missed flight due to gate change

I feel this is unacceptable. I teceived a ticket on Friday with a specific gate number. I sat for 40min at the gate. As you all know it is very noisy at the gate infront of the wimpy. They changed the gate at the last moment. I missed the flight. A flight was boarding 15minutes later from the same gate. Pleople starded queuing infront of this gate and i thought that i was in the correct line. I had to go to the counter i side the airport and asked to be placed on the flight boarding the woman at the counter laughed at me and said they only have a much later flight. Called her manager chatting for 10minutes having fun and then told me to pay another R800. I think this is unacceptable. I have paid almost R2000 for the original flight. I really want a refund on the extra payment.