expression() is not changing in the plot

I'm trying to write subscripts and superscripts in my ylab but is not working. I tried the code below but it appears exactly as it is in the code. I'm using the expression function. Any thoughts, I need help.

ggplot(gsw_mean_sd, aes(Day, average.ES, group=1))+
  geom_point()+
  geom_line()+
  labs(title="Stomatal conductance",x="Time (days)")+
  ylab(expression(G[sw]~(mol~m^-2~s^-1)))+
  theme_bw()

enter image description here