The amount of acquired gold in a Challenge Mode match is multiplied based on the number of trophies that you possess.

The total amount of gold acquired is equal to the base gold + bonus gold.
In other words, if your multiplier is x3, and you acquire 100 gold from Challenge Mode the awarded bonus gold will be 200 gold. In total you will receive 300 gold which is 3 times the amount of gold you earned (100 gold).