propane in its natural state is non-toxic, colourless, and odourless, the smell is added by the gas distributor as a safety measure for the consumer to detect leaks, this "smell" is also non-toxic and colourless. but your concern with propane is a common one;
will firing my gbb in a closet kill me? no, gbb's do not emit enough gas to do any harm to the body.
will smoking while firing a gbb start a fire or blow up? no, the gas expelled from the barrel after a shot is mixed with ambient air and cleaning fluids inside the pistol known as aeration, the concentration of propane is too little to actually start a fire from smoking.
is the smell bad for me? no, this is a chemical device used so a person can detect leaks or exposure to propane. the immediate danger is from the concentration of propane not the smell. you will begin to notice that after using your gbb's for a while that you will get used to the smell and it wont bother you much. atleast its true for me, I LOVE THAT SMELL... lol