This is the sort of saint that most non-Catholics find incomprehensible and even many Catholics find embarassing when they bother to think of such people at all. That's one reason why I love her.
Best described as a "victim soul," she was miraculously cured in childhood by the archangel Gabriel of spinal tuberculosis. Persistent health problems got her consistently rejected by religious orders, so she lived a life of prayer, service, and penance, first in her own family and then, from age 21, in the home of a another family who generously took her in after she was orphaned. She conversed regularly with her guardian angel and was eventually granted the "stigmata," ongoing physical manifestations of the five wounds of Christ on the Cross. Demons regularly harassed her, physically and spiritually. She died, exhausted, at 25.
Her apostolate was one of prayer and example. She had little education, never preached or wrote anything for public consumption, and had no following of spiritual devotees in her lifetime. But Satan hated her so much that he abandoned his customary strategy of stealth to attack her openly and observably. In other words, she's the complete opposite of me. That's the other reason I love her.
Saint Gemma is a patron of orphans and pharmacists. Her intercession is also efficacious for "temptations" in the classical sense of the term: trials of any kind, especially those that draw one toward sin.