How do you kill gnats that are in the lawn and tall grass?

The only thing I have found that will kill tall fescue or creeping fescue is "Roundup."tm

You build a weed wick (Google "weed wick applicator") that hangs in front of or behind whatever vehicle (ATV, mule, golf cart, tractor) you have that hangs horizontally about 12" above the desirable...

using chemicals hurts the plants around it....just cut it

I have observed that if the clippings fall in lumps on the lawn, the lumps will eventually turn live grass underneath brown in color and unable to grow. The clumps seem to occur when the grass is wet...

weed & feed depending on the type of grass. or MCPA on the weeds.