“ Both John the Baptist and archangel Gabriel are both important figures who practiced the religion.”
Ummm ... no.
They may be claimed to have been but Mohammad said Jesus taught Islam too so such claims are easily suspect.
“Both John the Baptist and archangel Gabriel are both important figures who practiced the religion.” How is it known? From the Britannica site. “The Mandaeans viewed Jesus as a false messiah but revered John the Baptist, who performed miracles of healing through baptism, which the Mandaeans viewed as a magical process giving immortality, purification, and physical health.”