Mets pitcher Jacob deGrom has a 0.56 ERA, the lowest in major league history through 10 starts, since earned runs became an official statistic in 1913.
The chart above is after 10 "appearances". After his 10th start in 1914, Dutch Leonard's ERA was 0.85 (he had made three relief appearances along the way). The highest Leonard's ERA got that year was 1.01 after a one-inning start (three earned runs) on July 25. deGrom's ERA has not yet risen above 0.80.
I understand including Gooden because of the Mets connection, but Pedro Martinez's 2000 season could have been included.