During 6 days my heating system consumed 11,4 m3 of natural gas keeping its temperature at 20 degrees of Celsius. No human or other energy source (except for refrigerator) was inside the flat at that time.
Outside temperature was about 0 degrees of Celsius (but I can provide more accurate temperature data set).
Is it possible to calculate energy efficiency of that flat by knowing how much energy was consumed to keep temperature?
No, because with this limited data you cannot separately estimate the three components that make up a building's energy signature:
Even if we assume the base load (i.e. the part of the energy used for other things than heating) is zero, there's no way you can estimate the base temperature (i.e. the outdoor temperature above which no heating is required).
If you assume that there were no casual gains during that period, then you can probably assume the base temperature to be 20 C, and only then can you reasonably claim that the energy efficiency of your building to be 11.4 * 1000 / (6 * 20) = 95 liters of gas per degree per day. Natural gas holds about 11 kWh per m3, so that's about 1.045 kWh / day / degree. That's very good (Swiss houses are between 4 and 10), whatever the size of your building, but it doesn't take into account your heating system's (in-)efficiency.
