Final answer:
The most likely way Gabriel contracted endocarditis is through the use of non-sterile needles during intravenous drug use, allowing bacteria to enter his bloodstream and infect the heart.
Step-by-step explanation:
The most likely way that Gabriel contracted endocarditis is through intravenous drug use. When using non-sterile needles, bacteria from the skin or other sources can be introduced directly into the bloodstream, bypassing the normal barrier defenses of the body. Given that Gabriel is a heroin addict, it is plausible that bacteria entered his bloodstream during drug injection, leading to the infection of the endocardium, which subsequently caused the heart murmur and endocarditis he is experiencing.