The bathroom is the most germ ridden area in the house. Turkish towels remain damp and the moisture allows bacteria to breed. As it is so thin the gamcha dries very fast so bacteria, germs and E. coli don’t multiply. Thus gamchas are safer to use than towels.
As they are so thin you can use a fresh gamcha every day. This is more hygienic than using one towel over a couple of days.
Gamchas take very little space in the washing machine as they are so thin. This puts less stress on the environment as you do fewer washing cylcles. So less water usage, less usage of power and less soapy water drained into the environment.
Hence gamchas are better for you and for the environment.