Siphoning gas by mouth is unlikely to kill you if you just get a little bit of gasoline in your mouth.
I used to siphon gas from my own vehicles as a teenager and younger adult and although I did get a bit of gasoline in my mouth I stopped sucking on the hose before enough gasoline got into my mouth to cause an issue.
I then immediately spit the gas out of my mouth and then used some mouth wash to wash my mouth out of the gasoline.
If you happen to swallow a good amount of gasoline then it could kill you.
I would advise though that if you need to siphon gasoline out of your own vehicles or equipment to get a siphon hose with a pump so that you don't have to risk your health.
Gasoline also tastes awful and it can take awhile to get the taste of gasoline out of your mouth.
Never steal gasoline from peoples vehicles or equipment because you could get shot or even serve jail or prison time and it's not right to steal.