There are a lot of things that could be causing the yellowing. Usually with under watering you would see some browning of the edges of the leaves or what we call scorching. Yellowing can also be caused by overwatering or poor drainage or both.
However, gazanias do not have a deep root system so they will require rather frequent watering in the summer months, perhaps once a day if you have sandy soils with good drainage.
I think you have to solve the watering problem first. Either get yourself an inexpensive water meter from the nursery, about $8, and use that to establish when to water or you can use the feel method if you have some gardening experience. With the feel method you dig down into the soil with a hand trowel around the root area, about six inches deep, pick up a small handful of soil and squeeze it into a clump. If the soil sticks together in a clump after bouncing it lightly in your hand, you will not need to water. If the clump falls apart after a couple of light bounces, then it is time to water. General rule of thumb is you need to water five times as often in the summer as you do in the winter.
The next is soil modification. Gazanias can handle our desert soils but they like it better if the soil is richer. Don't be afraid to add lots of compost, not mulch, into the soil at the time of planting. Mulch goes on top of the soil after planting. When you plant in our soils, you want to add a starter fertilizer, something high in phosphorus, in the backfill soil. Make sure the water is filling the hole as you are adding the backfill around the roots. This helps get rid of the air pockets.
After you are done, add an inch or two of mulch to the soil surface to keep the soil moist between waterings and keep the weeds in check. Handwater daily for the first week before turning it over to your irrigation system.
If these are on drip, you CANNOT give the plants enough water if the irrigation clock is set for just a few minutes. The drip emitters should be on for at least 30 minutes. Each of your gazanias should get about one gallon of water each time the emitters come on. So if this is a one gallon per hour emitter it needs to be on for one hour. If it is a two gallon per hour emitter then 30 minutes, etc.
By Robert Morris