Reason for specific location of a gnat swarm?
'Gnat' is the common name for a number of species of tiny flying insects in the Dipterid suborder Nematocera (https://entomology.ca.uky.edu/ef632), including Ceratopogonidae (biting midges), Chironomid midges (non-biting midges) and Simuliidae (blackflies). Like many other Diptera, a lot of these species mate aerially in swarms: the males gather using visual cues to determine swarm location. I'll try to add more detail to this later on why they go for specific locations, but in the meantime here are some papers on mating swarms in some of the groups normally called 'gnats':

General papers on swarming behaviour in Diptera:


Downes, J. A. 1969. The swarming and mating flight of Diptera. Annu.
Rev. Entomol. 14: 271–298.


Swarming behaviour of the family Ceratopogonidae:


Downes, J. A. 1955. Observations on the swarming flight and mating of
Culicoides (Diptera: Ceratopogonidae). Trans. R. Entomol. Soc. London
106: 213–236.
Campbell, M. M., and Kettle, D. S. 1979. Swarming of Culicoides
brevitarsis Kieffer (Diptera: Ceratopogonidae) with reference to
markers, swarm size, proximity of cattle, and weather. Australian. J.
Zool. 27: 17–30
Zimmerman, R. H., Barker, S. J., and Turner, E. C. 1982. Swarming and
mating behavior of a natural population of Culicoides variipennis
(Diptera: Ceratopogonidae). J. Med. Entomol. 19: 151–156.
Blackwell, A., Mordue, A. J., Young, M. R., and Mordue, W. 1992. The
swarming behavior of the scottish biting midge, Culicoides
impunctatus (Diptera, Ceratopogonidae). Ecol. Entomol. 17: 319–325.


Swarming behaviour of the family Chironomidae:


Gibson, N. H. E. 1945. On the mating swarms of certain Chironomidae
(Diptera). Trans. R. Entomol. Soc. London 95: 263–294.
Oliver, D. R. 1971. Life history of the Chironomidae. Annu. Rev.
Entomol. 16: 211–230.
Fyodorova, M. V., and Azovsky, A. I. 2003. Interactions between
swarming Chironomus annularius (Diptera: Chironomidae) males: role of
acoustic behavior. J. Insect Behav. 16: 295–306.


Swarming behaviour of the family Simuliidae:


Moorhouse, D. E., and Colbo, M. H. 1973. On the swarming of
Austrosimulium pestilens MacKerras and MacKerras (Diptera: Simuliidae). Australian J. Entomol. 12: 127–130
Hunter, D. M. 1979 Swarming, mating and resting behaviour of three species of black fly (Diptera: Simuliidae). Australian J. Entomol. 18: 1–6