How can I add fill gradient color to a density plot in R with only one variable

I have data that has two columns - date and values. I can create density plot with geom_density function. I would like to fill it with gradient color f.e. blue to red but I can't find how to do it. Anything I find on the internet requires Y value but I only have one variable.

Example of data:

Month_day; Temperature_anomaly
01-01;   -1.24
01-02;    0.31
01-03;    1.13
01-04;   -0.16

Code:

ggplot(data, aes(x = Temperature_anomaly)) + 
   geom_density() 

This creates simple density plot but have do I create single density plot and add gradient fill color based on the same value(Temperature_anomaly) to it?

An alternative solution using the {ggridges} package:

set.seed(123)
data <- data.frame(Temperature_anomaly = rnorm(20))

library(ggridges)
ggplot(data, aes(x = Temperature_anomaly, y = 1, fill = after_stat(x))) + 
  geom_density_ridges_gradient()+
  scale_fill_gradient2(low="blue", mid = "white", high="red")

which gives: