I don't think this account is accurate:
"Gerstenfeld (1915-1943) lived in Lvov and won the Polish Championship twice before reaching age 22. In 1939 his region of Poland became part of the USSR, which explains his participation in the 1940 USSR Championship."
Your source for this is Andrew Soltis, <Soviet Chess 1917-1991, p.131>, right?
According to Wikipedia, Guerstenfeld was never Polish chess champion, and certainly not Polish chess champion twice before age 22.
Lvov did indeed become part of the Ukraine SSR, but that alone doesn't explain his participation in the 1940 USSR Championship.
This is more accurately explained by the fact that Gurstenfeld shared first in this USSR Championship Semifinal in Kiev, 1940: