Answer: You didn't mention whether or not the pot you planted your strawberry guava in has drainage holes at the bottom. This is important, with or without the gravel you placed in the bottom of the pot. Without adequate drainage, the excess moisture will sour and will collect enough in the bottom of the pot to keep the potting soil too wet. If this is the case, repotting in fresh soil in a pot with good drainage will help save your plant. If the container does have lots of drainage holes in the bottom and this scenario doesn't apply, here are a few other possibilities for your new plant's poor performance:
If the plant was growing under shady conditions at the nursery and you repotted and then set it in full sunshine, it is going through some stress and will drop the leaves that developed in the shade in favor of growing new (and smaller) leaves that are more adapted to growing in full sun. If this is the case, it will eventually adjust and begin to look more attractive. You can move it to a spot with afternoon shade, or you can just wait for it to adjust to full sun and begin to look better.
A final thought - containers sitting in direct summer sunshine can become over-heated. This heat is transfered from the outside of the pot to the inside and can actually bake the roots of the plant. You can avoid this by grouping your plants together so they help shade one another, or you can place the pot behind a decorative rock or other object - anything that might deflect the sun's rays.
Hope this information helps you determine why your strawberry guava looks unhappy.
Best wishes with your garden!
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