I asked the paint shop about this, they had no idea. I was getting desperate and I started to try things to make the smell go away:
- air purifier - no effect
- water with sodium bicarbonate spray - the smell was going away for a few hours then it was getting back
- cut onions - after this the smell was not so strong but still present
- vinegar spray - it was now smelling of vinegar instead of gas, and it was not so annoying but it didn't last too much
- candles - I read somewhere that fire neutralizes some substances causing smells, but I didn't notice improvements
- essential oil - I think this had a very strong and positive effect, it was the last thing I tried and within days the smell was gone completely
As you can see I have tried many things and I'm not sure which of them helped, but the important thing is the walls no longer stink.