The leaves of my Gazania plant turn yellow and the plant dies.
This would usually indicate a nitrogen deficiency. The thing is... It is rare to see a deficiency in these plants unless they are in rocky or sandy soil. It could also be that the pH of the soil is too high. Iron sulfate will bring this down a bit. This will help the nitrogen become available to the plant again.
Here is an article on their care: https://www.gardeningknowhow.com/ornamental/flowers/gazania/gazania-treasure-flowers.htm