I will be grateful if anyone could tell me critical speeds of GE 6FA model.

 

https://ncalculators.com/mechanical/rayleigh-ritz-method-critical-speed-calculator.htm

https://www.calculatoratoz.com/en/static-deflection-of-the-shaft-calculator/Calc-3758

Here you have it!
These are all necessary informations to calculate elaborate critical speed of any turbine shaft ...even That wonderful frame6FA MADE BY Belfort GEEPE...

Any time!
Cheers
James