It sounds like you are dealing with Prostrate Spurge, a common weed with small leaves, milky sap and hundreds of seeds.

The gnats you describe are actually Whiteflies. The adults do nothing to harm plants but their larvae sip fluids from plant leaves, especially hibiscus and lantana. As soon as we get our first good frost they will be gone, but in the meantime you can use a soapy water spray applied to the underside of the leaves. This will kill any adults and larvae that are clinging there.

Adult whiteflies are attracted to the color yellow. Sticky traps can be placed in your landscape where the insects become lodged and therefore can't lay eggs on your plants. You can also make your own sticky traps with a plastic yellow plate and vaseline. Staple the plate onto a wooden stake and place near affected plants.

For your spurge weeds, use a contact herbicide to kill mature plants and a pre-emergent spray applied to the soil that will keep any leftover seeds from sprouting.

For more information on spurge visit http://www.ipm.ucdavis.edu/PMG/WEEDS/spotted_spurge.html
Good luck

Here is a link that might be useful: Whiteflies