What is air fuel ratio of gas turbine engine?

Depends on the fuel. Type of engine is not important.

For gasoline, a 16:1 ratio would be ideal, for alcohol (Ethanol) 8:1 is considered ideal.

A good estimate for any fuel is to split the difference between Lower Explosive Limit (won't burn; too lean) and Upper Explosive Limit (won't burn, too rich). For example, Propane has a LEL of 24,000 ppm, and HEL of 96,000 ppm, so split the difference and 60,000 ppm should burn pretty well.

60,000 ppm should be about 6% or 17:1.