The best way to collect gold is by having more habitats. Each time you collect gold you have a chance to collect an item! You also have a higher chance to collect gold when your habitat is full of gold. This is visible when the gold icon is no longer see-through. Collecting gold is one of the easiest items to collect for the runner island. Having more habitats = more chances to get items from collecting gold.
Another way to quickly increase the amount of gold you can collect is to power up your dragons You can save your powerups for the tasks in the runner island that require you to power up dragons. But you should prioritise dragons that give the most gold per minute
Note that once a dragons level increases above level 10 - the increase in gold / level halves. Your rarest dragons usually will pay the most gold per minute.
You can also try and focus on getting as many terra habitats as possible, but itsnt the best long term strategy, because its not great once the runner island event ends!