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Question:  What type of SS is suitable for handling of HF gas, 304L or 316L? Can SS 304L be used at low temperature i.e. at -20 Celcius?

Answer: 1) If the gas is dry and no water condensation is possible, also during shutdown of the plant, then is possible to use a
Stainless Steel, but you've to be absolutely sure that water will never be present.
As you can see in this article :
Selection of stainless steels for handling hydrofluoric acid (HF)
http://www.bssa.org.uk/topics.php?article=8

stainless steel is not suitable to handle HF even at very low conc and temperature.

2) SS 304 can be used down to very low temp approx -200°C.
"Toughness. Austenitic stainless steels are inherently tough, maintaining the ductile fracture mode and high absorbed energy in impact tests to cryogenic temperatures
(-200°C)."
ref:http://www.australwright.com.au/datasheets/alloys/Stainless%20Steels/304%20SS...
or you can check on asme or astm code.